Printing device, printing device control program, printing device control method, print data generation device, print data generation program, and print data generation method

ABSTRACT

A device including: an acquirer acquiring image data having an M-value (M≧3) pixel value; a memory storing nozzle information; another memory storing error diffusion matrices having different diffusion ratios of an error; a selector selecting specific pixel data in the image data; an N-value converter converting the M-value to an N value (M&gt;N≧2); a diffuser diffusing the error to pixel data not subjected to the conversion in the image data according to the error diffusion matrix by using a difference between a selected pixel data value before and after conversion as the error; a generator generating print data defining nozzle dot information corresponding to the image data after the conversion; and a printer. The diffuser selects a specific error diffusion matrix for each selected pixel data according to the nozzle information, and diffuses the error to the pixel data not subjected to the conversion.

RELATED APPLICATIONS

This application claims priority to Japanese Patent Application Nos.2005-020834 filed Jan. 28, 2005 and 2005-279404 filed Sep. 27, 2005which are hereby expressly incorporated by reference herein in theirentirety.

BACKGROUND

1. Technical Field

The present invention relates to a printing device used in a facsimilemachine, a copying machine, and office automation equipment, a printingdevice control program, and a printing device control method, and moreparticularly, to a so-called ink-jet printing device configured to drawspecific characters and images by ejecting fine particles of liquid inksof several colors onto a print sheet (recording member), a printingdevice control program, a printing device control method, a print datageneration device, a print data generation program, and a print datageneration method.

2. Related Art

Hereinafter, a printing device, in particular, a printer adopting theink jet method (hereinafter, referred to as an ink-jet printer) will bedescribed.

Ink-jet printers have been used extensively among not only offices butalso general users as personal computers and digital cameras have comeinto widespread use because they are generally inexpensive andhigh-quality color printed matter can be obtained easily.

An ink-jet printer normally produces desired printed matter by moving amobile body, called a carriage and provided with an ink cartridge and aprint head in a single-piece design, to reciprocate over a medium usedfor printing (for example, a print sheet) in a direction perpendicularto the paper transportation direction, and simultaneously ejecting(discharging) dot-shaped particles of liquid inks from the nozzlesprovided to the print head to draw specific characters and images on themedium. By providing the carriage with ink cartridges of four colorsincluding black (black, yellow, magenta, and cyan) and the print headsfor respective colors, not only monochrome printing but also full colorprinting can be readily executed by combining the respective colors(color printing of six, seven, or eight colors is now available by usinglight cyan, light magenta, etc. in addition to the four colors).

In an ink-jet printer of a type that executes printing by moving theprint heads on the carriage to reciprocate in a direction perpendicularto the paper transportation direction, the print heads have toreciprocate several tens to a hundred times or more to clearly print afull page. Hence, it has a drawback that printing takes a considerablylong time in comparison with a printing device adopting another method,such as a laser printer using electrophotographic technology such as ina copying machine.

Meanwhile, an ink-jet printer of a type that omits the carriage bydisposing a long print head of a size comparable to (or larger than) thewidth of the print sheet is known. Because there is no need to move theprint head in the width direction of the print sheet in this ink-jetprinter, a so-called single-scan (1-pass) printing can be achieved,which in turn enables printing as fast as a laser printer. In addition,the carriage to mount the print heads thereon and a driving system tomove the carriage can be omitted. The housing of the printer cantherefore be reduced in size and weight; moreover, there is anotheradvantage that the printer becomes remarkably quiet. Incidentally, theink-jet printer of the firstly-mentioned type is generally referred toas “a multi-pass printer”, and the ink-jet printer of thesecondly-mentioned type is generally referred to as “a line-headprinter” or “a serial printer”.

The print head indispensable to the ink-jet printer is provided withfine nozzles having a diameter of about 10 to 70 μm that are aligned atregular intervals in one line or in lines in more than one row along theprinting direction. This configuration, however, may possibly give riseto a so-called flight deviation (or flight bend) phenomenon, which is anevent that the ink ejecting direction of a part of the nozzles isinclined due to a manufacturing error or the landing position of a dotformed by a given nozzle is displaced from the ideal position becausethis nozzle is disposed at a position displaced from the ideal position.In addition, the characteristic varies from nozzle to nozzle, and for anozzle having a considerable variance, a quantity of ink becomes toolarge or too small in comparison with an ideal quantity.

Defective printing referred to as the so-called banding (streak)phenomenon thus occurs in a portion printed by such failing nozzles,which possibly deteriorates the printing quality. In other words, oncethe flight deviation phenomenon occurs, a distance between dots ejectedfrom adjacent nozzles becomes uneven. In a portion where the distancebetween adjacent dots becomes larger than usual, a white streak (when aprint sheet is white) occurs, and in a portion where the distancebetween adjacent dots becomes smaller than usual, a dark streak occurs.Also, when the value of a quantity of ink does not coincide with theideal value, a dark streak occurs in a portion printed by a nozzlehaving a larger quantity of ink and a white streak occurs in a portionprinted by a nozzle having a smaller quantity of ink.

In particular, the banding phenomenon noticeably occurs more readilywith a line-head printer in which the print head or a medium used forprinting is fixed (1-pass printing) than with the multi-pass printer(serial printer) (for the multi-pass printer, a technique has beendevised to make the banding less noticeable by exploiting itsconfiguration to move the print head to reciprocate a number of times).

Such being the case, studies and developments have been conductedassiduously for hardware, such as improvements in the manufacturingtechnique and improvements in the design of the print head, to prevent akind of defective printing caused by the banding phenomenon. However, itis difficult to provide a print head that is 100% free from the bandingphenomenon because of the limits to hardware improvement due tomanufacturing costs and current technical abilities.

Hence, as a technique for suppressing the banding phenomenon, printingcontrol by software as described below is now used in addition to theimprovements in the hardware as described above.

According to a first technique disclosed in the related art, forexample, in JP-A-2002-19101 and JP-A-2003-136702, variances amongnozzles and an ink ejection failure are addressed as follows. That is,variances among heads are addressed using a shading correction techniquein a less dense portion, whereas the banding or variances are made lessnoticeable using an alternate color to the original color (for example,in the case of printing in black, cyan or magenta is used as analternate color to black) in a dense portion.

According to a second technique disclosed in the related art, forexample, in JP-2003-63043, a quantity of ejected ink is increased foradjacent nozzles corresponding to pixels in the neighborhood of anon-ejecting nozzle for a solid image (an image portion having arelatively large area in comparison with a line image, and it is aregion densely covered with ink; however, there may be a case where itis not fully covered due to an edge effect), so that a solid image isformed using the nozzles as a whole.

According to a third technique disclosed in the related art, forexample, in JP-A-5-30361, the banding phenomenon is avoided by absorbinga variance in the quantity of ink ejected from the nozzles by feedingback a quantity of variance in each nozzle to error diffusion.

According to a fourth technique disclosed in the related art, forexample, in JP-A-2004-58284, assuming that there is a nozzle (N) underan abnormal ink ejection condition, record data corresponding to thisfailing nozzle (N) is appended to record data corresponding toneighboring nozzles (N−1) and (N+1) positioned in the neighborhood ofthe failing nozzle (N). The banding phenomenon can be avoided bycompensating for the record data corresponding to the abnormal nozzle(N) in this manner.

However, the first technique in the related art for suppressing thebanding phenomenon and variances using another color has a problem thatthe hue changes in a portion where this technique is applied. Thistechnique is therefore unsuitable for printing that requires a highimage quality and a high quality, such as printing of a color pictureimage.

In addition, when the method of avoiding the white streak phenomenon bydistributing the information about the non-ejecting nozzle to nozzles onthe right and left is adopted as a countermeasure against the flightdeviation phenomenon, it is indeed possible to reduce a white streak;however, there is a problem that banding still occurs in a denseportion.

The second technique in the related art has no problem when a solidimage is produced as printed matter. However, this technique cannot beused when halftone printed matter is produced. In addition, the methodof filling fine lines using another color has no problem for use to alimited extent. However, for an image on which the color used as analternate color is used continuously, the hue changes in part of theimage. This technique therefore has the same problem as the firsttechnique in the related art.

The third technique in the related art has difficulty solving a problemthat the dot formation content has discrepancies, because complicatedprocessing is required to enable appropriate feedback for solving such aproblem.

The fourth technique in the related art has a problem that in a casewhere dots of different sizes are formed using nozzles in theneighborhood in the downstream processing after binarization, when thedots have a γ characteristic, there is a risk that the area grayscale islost in the portion where this technique is applied.

SUMMARY

One advantage of some aspects of the invention is to provide a novelprinting device, printing device control program, printing devicecontrol method, print data generation device, print data generationprogram, and print data generation method, wherein each can eliminate ormake deterioration of print image quality almost unnoticeable.

Another advantage of some aspects of the invention is to provide a novelprinting device, printing device control program, printing devicecontrol method, print data generation device, print data generationprogram, and print data generation method, wherein each can eliminate ormake deterioration of print image quality resulting from the bandingphenomenon attributed to the flight deviation phenomenon almostunnoticeable.

A further advantage of some aspects of the invention is to provide anovel printing device, printing device control program, printing devicecontrol method, print data generation device, print data generationprogram, and print data generation method, wherein each can eliminate ormake deterioration of print image quality resulting from an ink ejectionfailure almost unnoticeable.

According to a first aspect of the invention, a printing device thatprints an image on a medium used for printing using a print head havingnozzles capable of forming dots includes: an image data acquisitionportion that acquires image data having an M-value (M≧3) pixel value; anozzle information storage portion that stores nozzle informationindicating a characteristic of each nozzle; an error diffusion matrixstorage portion that stores plural kinds of error diffusion matriceshaving different diffusion ratios of an error assigned to pixel data asdiffusion targets; a pixel data selecting portion that selects specificpixel data in the image data; an N-value conversion processing portionthat performs N-value conversion processing that is processing to covertthe M-value (M≧3) pixel value indicated by the selected pixel data to anN (M>N≧2) value; an error diffusion portion that diffuses the error topixel data that has not been subjected to the N-value conversionprocessing in the image data according to the error diffusion matrix byusing a difference between a value of the selected pixel data and avalue of the selected pixel data after the N-value conversion processingas the error, thereby updating pixel values in the image data; a printdata generation portion that generates print data that definesinformation about a dot formation content of the nozzles correspondingto the image data after the N-value conversion processing; and aprinting portion that prints the image on the medium using the printhead according to the print data. The error diffusion portion selects aspecific error diffusion matrix for each selected pixel data from theerror diffusion matrix storage portion according to the nozzleinformation, and diffuses the error to the pixel data that has not beensubjected to the N-value conversion processing according to the selectederror diffusion matrix.

When configured in this manner, the image data acquisition portion isable to acquire image data having an M-value (M≧3) pixel value. Thenozzle information storage portion is able to store nozzle informationindicating the characteristic of each nozzle. The error diffusion matrixstorage portion is able to store plural kinds of error diffusionmatrices having different diffusion ratios of the error assigned topixel data as diffusion targets. The pixel data selecting portion isable to select specific pixel data in the image data. The N-valueconversion processing portion is able to perform the N-value conversionprocessing that is the processing to convert the M-value (M≧3) pixelvalue indicated by the selected pixel data to an N (M>N≧2) value. Theerror diffusion portion is able to diffuse the error to pixel data thathas not been subjected to the N-value conversion processing in the imagedata according to the error diffusion matrix by using a differencebetween the value of the selected pixel data and the value of theselected pixel data after the N-value conversion processing as theerror, thereby updating pixel values in the image data. The print datageneration portion is able to generate print data that definesinformation about a dot formation content of the nozzles correspondingto the image data after the N-value conversion processing. The printingportion is able to print the image on the medium using the print headaccording to the print data.

Further, the error diffusion portion selects a specific error diffusionmatrix for each selected pixel data from the error diffusion matrixstorage portion according to the nozzle information, and diffuses theerror to the pixel data that has not been subjected to the N-valueconversion processing according to the selected error diffusion matrix.

It is thus possible, for example, to perform the error diffusionprocessing by switching to an error diffusion matrix of a kindappropriate to avoid the occurrence of the banding phenomenon as neededfor each factor that causes the banding phenomenon of various kinds, forexample, when suppressing deterioration of the printing quality causedby a white streak or a dark streak resulting from the banding phenomenonchiefly attributed to the characteristic of nozzles that occurs due toan ink ejection failure of a nozzle or the flight deviation phenomenonof a nozzle by which the dot forming position is displaced from theideal position, or when suppressing deterioration of the printingquality, such as a white streak and a dark streak resulting from thebanding phenomenon attributed to the γ characteristic of a dot formed bya nozzle. Hence, there can be achieved an advantage that deteriorationof the printing quality, such as a white streak and a dark streakresulting from the banding phenomenon can be suppressed in anappropriate manner for each factor that causes the banding phenomenon.

The dot referred to herein means a single region on a print mediumformed by ink that is ejected from one or more than one nozzle and landson the print medium. It goes without saying that the area of the dot isnever “0”, and the dot has a certain size (area); moreover, plural kindsof dots that differ in size are present. It should be noted, however,that a dot formed by ejecting ink does not necessarily form a perfectcircle. For example, when a dot is formed in a shape other than aperfect circle, for example, in an elliptical shape, an average diameteris deemed as the dot diameter. Alternatively, an equivalent dot in theshape of a perfect circle having an area equal to an area of a dotformed by ejecting a specific quantity of ink is assumed, and thediameter of the equivalent dot is deemed as the dot diameter. To strikedots having different densities separately, for example, methods asfollows are possible: striking dots having the same size and differentdensities, striking dots having the same density and different sizes,and overstriking dots having the same density and different quantitiesof ejected ink to vary the densities. In a case where one ink dropletejected from one nozzle splits and lands on the medium, it is assumedthat a single dot is formed. In a case where two or more dots formed bytwo nozzles or one nozzle with a time interval unite with each other, itis assumed that two dots are formed. The same applies to aspectsrelating to a printing device control program, aspects relating to aprinting device control method, aspects relating to a print datageneration device, aspects relating to a print data generation program,aspects relating to a print data generation method, aspects relating torecording media having recorded the programs, and the description in theDescription of Exemplary Embodiments column below.

The image data acquisition portion acquires image data inputted from anoptical printing result read portion, such as a scanner, or passively oractively acquires image data stored in an external device via a network,such as the LAN or the WAN. Alternatively, the image data acquisitionportion acquires image data from a recording medium, such as a CD-ROMand a DVD-ROM, via a driving device, such as a CD drive and a DVD driveequipped to the printing device, or acquires image data stored in thestorage device equipped to the printing device. In short, theacquisition involves at least the input, obtainment, reception, andreading. The same applies to aspects relating to a printing devicecontrol program, aspects relating to a printing device control method,aspects relating to a print data generation device, aspects relating toa print data generation program, aspects relating to a print datageneration method, aspects relating to recording media having recordedthe programs, and the description in the Description of ExemplaryEmbodiments column below.

The nozzle information storage portion stores the nozzle informationincluding information about a quantity of the flight deviation andinformation indicating the absence or presence of an ejection failure ofeach nozzle by any means at any time. The nozzle information storageportion may have stored the nozzle information previously, or it maystore the nozzle information by an input from the outside when theprinting device is activated without having pre-stored the nozzleinformation. The nozzle information can be stored at any timing as longas it is the timing at which the printing device is restored to a statestored during use. For example, before the printing device comes ontothe market as a product, such as at the time of shipment from thefactory, a quantity of displacement of the dot formation position and anink ejection condition of nozzles forming the print head may be checkedfrom the printing result by the print head using an optical printingresult read portion, such as a scanner, so that the checking result ispre-stored as the nozzle information. Alternatively, a quantity ofdisplacement of the dot formation position and an ink ejection conditionof nozzles forming the print head may be checked during the use of theprinting device in the same manner as the checking at the time ofshipment from the factory, so that the checking result is stored as thenozzle information. Further, a quantity of displacement of the dotformation position and an ink ejection condition of the print head maybe checked after the use of the printing device from the printing resultby the print head using an optical printing result read portion, such asa scanner, periodically or at a predetermined time to address a changeof the characteristic of the print head, so that the nozzle informationcan be updated by storing the checking result together with the data atthe time of shipment from the factory or by writing the checking resultover the data at the time of shipment. The same applies to aspectsrelating to a printing device control program, aspects relating to aprinting device control method, aspects relating to a print datageneration device, aspects relating to a print data generation program,aspects relating to a print data generation method, aspects relating torecording media having recorded the programs, and the description in theDescription of Exemplary Embodiments column below.

The information about the dot formation content of the nozzles is madeof information needed when dots are formed by a nozzle, such asinformation about the absence or presence of dots (whether dots areformed by the nozzle) and information about the sizes of dots when dotsare formed (for example, one of three sizes: large, medium, and small)for each pixel value in the image data. For example, when there is onlyone forming size, it may be made of information about the absence orpresence of dots alone.

As has been described, the banding phenomenon includes a printingfailure in which a white streak and a dark steak occur simultaneously inthe printing result due to the so-called flight deviation phenomenoncaused by a nozzle whose dot forming position is displaced from theideal forming position, a printing failure in which a white streak or adark streak occurs in the printing result due to an inadequate quantityof ejected ink caused by a non-ejecting nozzle, and a printing failurein which a white streak or a dark streak occurs in the printing resultdue to the γ characteristic of a dot formed by the nozzle. The sameapplies to aspects relating to a printing device control program,aspects relating to a printing device control method, aspects relatingto a print data generation device, aspects relating to a print datageneration program, aspects relating to a print data generation method,aspects relating to recording media having recorded the programs, andthe description in the Description of Exemplary Embodiments columnbelow.

A white streak means a portion (region) in which a phenomenon that adistance between adjacent dots becomes wider than a specific distancedue to the flight deviation phenomenon occurs continuously, which makesthe color of the background of the print medium to appear noticeably asa streak. Likewise, a dark streak means a portion (region) in which aphenomenon that a distance between adjacent dots becomes shorter than aspecific distance due to the flight deviation phenomenon occurscontinuously, which makes the color of the background of the printmedium invisible, or the color apparently becomes darker as the distancebetween dots becomes shorter, or a portion (region) where part of dotsformed at displaced positions overlap on dots formed normally and theoverlapped portion appears noticeably as a dark streak. A white streakmay occur because of a nozzle having an insufficient quantity of ink,whereas a dark streak may occur because of a nozzle having an excessivequantity of ink. The same applies to aspects relating to a printingdevice control program, aspects relating to a printing device controlmethod, aspects relating to a print data generation device, aspectsrelating to a print data generation program, aspects relating to a printdata generation method, aspects relating to recording media havingre-corded the programs, and the description in the Description ofExemplary Embodiments column below.

The error diffusion matrix means a matrix having stored information(relative position information) indicating the position (diffusiondirection) of pixel data as diffusion targets with respect to the pixeldata to be diffused and information about diffusion ratios assigned tothe respective pixel data as diffusion targets, which is used when theM-value pixel data is converted to an N value by the N-value conversionprocessing and subsequently, so that a difference (error) between an Nvalue after the conversion and an M value before the conversion isdiffused (distributed) to pixel data that has not been subjected to theN-value conversion processing in the neighborhood of the pixel data ofinterest (this is generally referred to as the error diffusion method).There are plural kinds of error diffusion matrices, such as matriceshaving different shapes, matrices having different sizes (differentnumbers of diffusion targets), and matrices having different diffusionratios. The same applies to aspects relating to a printing devicecontrol program, aspects relating to a printing device control method,aspects relating to a print data generation device, aspects relating toa print data generation program, aspects relating to a print datageneration method, aspects relating to recording media having recordedthe programs, and the description in the Description of ExemplaryEmbodiments column below.

The diffusion direction means, for example, a direction of a weightvector of the pixel of interest (selected pixel) according to the errordiffusion matrix. In order to describe the weight vector, a fundamentalvector is set first. As is shown in FIG. 22A, given the origin (0, 0) asthe coordinate of the pixel of interest, (1, 0) as the coordinate of thepixel immediately to the right of the pixel of interest, and (0, 1) asthe coordinate of the pixel immediately below the pixel of interest.Then, the fundamental vector of the pixel of interest can be expressedby Equations (1) and (2) below (shown also in FIG. 22A).{right arrow over (a)}=(1,0)   (Equation 1){right arrow over (b)}=(0,1)   (Equation 2)

When the pixel of interest shown in FIG. 22A is converted to an N valueaccording to the error diffusion matrix shown in FIG. 22B, a differencebetween the pixel value of the pixel of interest after the N-valueconversion and the pixel value of the pixel of interest before theN-value conversion is calculated as an error. The error is then diffusedto the neighboring pixels present at positions defined in the errordiffusing matrix with weights defined in the error diffusion matrixshown in FIG. 22B (numerical values written in the cells immediately tothe right, immediately below, at diagonally lower left, and atdiagonally lower right with respect to the cell of the pixel of interestin FIG. 22B). The ratios (numerical values shown in the respectivecells) of the error to be diffused to the neighboring pixels arereferred to as the error diffusion ratios.

The weight vector is the fundamental vector multiplied with each weight.The weight vectors for pixels (1) through (4) shown in FIG. 22C areexpressed by Equations (3) through (6) below, respectively. To be morespecific, the fundamental vector multiplied by 7/16, which is the weightassigned to the pixel (1) shown in FIG. 22B, is the weight vectorexpressed by Equation (3) below. Likewise, the fundamental vectormultiplied by 3/16, which is the weight assigned to the pixel (2), isthe weight vector expressed by Equation (4) below. The fundamentalvector multiplied by 5/16, which is the weight assigned to the pixel(3), is the weight vector expressed by Equation (5) below. Thefundamental vector multiplied by 1/16, which is the weight assigned tothe pixel (4), is the weight vector expressed by Equation (6) below. ->= 7 / 16 ⁢   ⁢ a → ( Equation ⁢   ⁢ 3 ) ( Equation ⁢   ⁢ 4 ) ( Equation ⁢   ⁢ 5) ( Equation ⁢   ⁢ 6 )

The diffusion direction is a direction indicated by a sum (the weightvector of the entire error diffusion matrix) of the weight vectorsexpressed by Equations (3) through (6) above, which is expressed byEquation (7) below. M → = -> + -> + -> + -> = 5 / 16 ⁢   ⁢ a → + 9 / 16 ⁢  ⁢b → ( Equation ⁢   ⁢ 7 )

The opposite direction means a direction in which the directionindicated by the weight vector of the entire error diffusion matrixexpressed by Equation (7) above is directly opposite. The oppositedirection establishes a relation expressed by Equation (8) below betweenthe weight vector of the entire error diffusion matrix and the directlyopposite weight vector of the pixel of interest.{right arrow over (M)}′={right arrow over (αm)}  (Equation 8)

In other words, the weight vector expressed by Equation (7) abovemultiplied by a negative constant α is the weight vector having thedirectly opposite vector direction. The same applies to aspects relatingto a printing device control program, aspects relating to a printingdevice control method, aspects relating to a print data generationdevice, aspects relating to a print data generation program, aspectsrelating to a print data generation method, aspects relating torecording media having recorded the programs, and the description in theDescription of Exemplary Embodiments column below.

According to a second aspect of the invention, in the printing device ofthe first aspect, the nozzle information includes ejection failureinformation indicating absence or presence of an ink ejection failurefor each nozzle.

When configured in this manner, a nozzle having an ink ejection failurecan be readily identified. Hence, by preparing error diffusion matricessuitable for avoiding the banding phenomenon caused by an ink ejectionfailure, and performing the error diffusion processing by selecting anappropriate error diffusion matrix as needed, there can be achieved anadvantage that deterioration of the printing quality, such as a whitestreak and a dark streak resulting from the banding phenomenonattributed to an ink ejection failure, can be suppressed appropriately.

An ink ejection failure means a state where a nozzle is incapable ofejecting ink in an ideal manner, for example, in a case where the nozzleis totally incapable of ejecting ink, a quantity of ejected ink isinsufficient, a quantity of ejected ink is excessive, or the nozzle isnot able to eject ink at the ideal position.

The absence or presence of an ink ejection failure with the nozzle canbe detected, for example, by a CCD sensor provided to the printingdevice. Information indicating the absence or presence of an inkejection failure can be therefore generated according to the detectionresult. The same applies to aspects relating to a printing devicecontrol program, aspects relating to a printing device control method,aspects relating to a print data generation device, aspects relating toa print data generation program, aspects relating to a print datageneration method, aspects relating to recording media having recordedthe programs, and the description in the Description of ExemplaryEmbodiments column below.

According to a third aspect of the invention, in the printing device ofthe second aspect, the nozzle information includes information about aquantity of position displacement between an actual forming position ofa dot by each nozzle and an ideal (e.g., desired) forming position ofthe dot.

When configured in this manner, not only is it possible to identify anozzle that causes the so-called flight deviation phenomenon occurringwhen the dot forming position is displaced from the ideal formingposition with ease, but it is also possible to examine the magnitude ofa quantity of the flight deviation. Hence, by preparing error diffusionmatrices suitable for avoiding the banding phenomenon caused by theflight deviation phenomenon, and performing the error diffusionprocessing by selecting an appropriate error diffusion matrix as needed,there can be achieved an advantage that deterioration of the printingquality, such as a white streak and a dark streak resulting from thebanding phenomenon attributed to the flight deviation phenomenon, can besuppressed appropriately.

According to a fourth aspect of the invention, in the printing device ofany one of the first through third aspects, the error diffusion matrixincludes a diagonally weighted error diffusion matrix that is an errordiffusion matrix in which the diffusion ratio of the error assigned tothe pixel data corresponding to pixels as diffusion targets positionedin a diagonal direction, which is a direction other than a pixelselecting direction and a direction perpendicular to the pixel selectingdirection, is made larger than the diffusion ratio assigned to the pixeldata as diffusion targets positioned in a direction other than thediagonal direction.

When configured in this manner, the error diffusion processing isenabled with the use of the diagonally weighted error diffusion matrixto diffuse the error more to the pixel data disposed in the diagonallydirection in the image data. Because a change in contrast in thediagonal direction is hardly perceived visually, by performing the errordiffusion processing using the diagonally weighted error diffusionmatrix, there can be achieved an advantage that deterioration of theimage quality in the diagonal portion resulting from the error diffusionprocessing can be suppressed.

According to a fifth aspect of the invention, in the printing device ofthe fourth aspect, in the error diffusion processing for the error ofpixel data corresponding to at least one of a nozzle having a quantityof the position displacement equal to or larger than a specific quantityand a nozzle in a neighborhood, the error diffusion portion diffuses theerror of the pixel data to the pixel data that has not been subjected tothe N-value conversion processing using the diagonally weighted errordiffusion matrix.

When configured in this manner, the error diffusion processing isenabled with the use of the diagonally weighted error diffusion matrixto diffuse the error more to the pixel data disposed in the diagonaldirection in the image portion corresponding to a nozzle causing thebanding phenomenon attributed to the flight deviation phenomenon. Hence,there can be achieved an advantage that not only is it possible toeliminate a white streak and a dark streak resulting from the bandingphenomenon attributed to the flight deviation phenomenon or make suchstreaks almost unnoticeable, but it is also possible to suppressdeterioration of the image quality caused by the error diffusionprocessing.

According to a sixth aspect of the invention, in the printing device ofthe fourth or fifth aspect, in the error diffusion processing for theerror of pixel data corresponding to at least one of a nozzle having anink ejection failure and a nozzle in a neighborhood, the error diffusionportion diffuses the error of the pixel data to the pixel data that hasnot been subjected to the N-value conversion processing using thediagonally weighted error diffusion matrix.

When configured in this manner, the error diffusion processing isenabled with the use of the diagonally weighted error diffusion matrixto diffuse the error more to the pixel data disposed in the diagonaldirection in the image portion corresponding to a nozzle causing thebanding phenomenon attributed to an ink ejection failure. Hence, therecan be achieved an advantage that not only is it possible to eliminate awhite streak and a dark streak resulting from the banding phenomenonattributed to an ink ejection failure or make such streaks almostunnoticeable, but it is also possible to suppress deterioration of theimage quality caused by the error diffusion processing.

According to a seventh aspect of the invention, in the printing deviceof any one of the third through fifth aspects, the N-value conversionprocessing portion converts the pixel value of the pixel datacorresponding to a nozzle having an ink ejection failure to a valueclose to one of a lowest density value and a lowest luminance value, andthe error diffusion portion diffuses the error to the pixel data thathas not been subjected to the N-value conversion processing in aneighborhood of the pixel data after conversion using the errordiffusion matrix by using a difference between the pixel value of thepixel data before the conversion and the pixel value of the pixel dataafter the conversion as the error.

When configured in this manner, not only is it possible to prevent anozzle having an ink ejection failure from forming a dot, but it is alsopossible to diffuse the pixel value as an error to the pixel data in theneighborhood. It is thus possible to compensate for the pixel data as tothe grayscale value of a portion corresponding to the nozzle having anejection failure by the pixel data in the neighborhood. Hence, there canbe achieved an advantage that a loss of the area grayscale caused by anink ejection failure of the nozzle can be prevented.

The lowest density value means a value at which the density reaches thelowest within the grayscale values of an image. For example, in a casewhere the grayscale of an image is expressed by 8 bits (0 to 255), thelowest density value is “255” when the pixel value is the luminancevalue and “0” when the pixel value is the density value. The sameapplies to aspects relating to a printing device control program,aspects relating to a printing device control method, aspects relatingto a print data generation device, aspects relating to a print datageneration program, aspects relating to a print data generation method,aspects relating to recording media having recorded the programs, andthe description in the Description of Exemplary Embodiments columnbelow.

A value close to the lowest density value means a density value suchthat an individual cannot perceive a corresponding portion when he seesan image locally. For example, when dots of such a density value areformed, dots of a small size, such as those of the smallest size andthose of the second smallest size, are formed. The same applies toaspects relating to a printing device control program, aspects relatingto a printing device control method, aspects relating to a print datageneration device, aspects relating to a print data generation program,aspects relating to a print data generation method, aspects relating torecording media having recorded the programs, and the description in theDescription of Exemplary Embodiments column below.

According to an eighth aspect of the invention, in the printing deviceof any one of the first through seventh aspects, the print head is aprint head in which the nozzles are aligned continuously across one of arange as large as a placement region of the medium and a range largerthan the placement region (i.e., a width at least as large as theplacement region of the medium).

When configured in this manner, as has been described, there can beachieved an advantage that print data effective to make a white streakand a dark streak resulting from the banding phenomenon that readilyoccurs particularly with the use of a line-head print head thatcompletes printing by the so-called 1-pass operation less noticeable.

According to a ninth aspect of the invention, in the printing device ofany one of the first through seventh aspects, the print head is a printhead that executes printing while moving in a direction orthogonal to apaper transportation direction of the medium.

The banding phenomenon is observed noticeably with the line-head printhead; however, it also occurs with the multi-pass print head. Hence, byapplying the printing method in any one of the first through seventhaspects to the multi-pass print head, there can be achieved an advantagethat print data effective to make a white streak and a dark streakresulting from the banding phenomenon occurring also in the multi-passprint head less noticeable.

In the case of the multi-pass print head, the banding phenomenon can beavoided by devising a technique for scanning the print headrepetitively. However, when the printing device of any one of the firstthrough seventh aspects is used, the need to scan the print head overthe same portion a number of times can be eliminated. Faster printing isthus enabled.

According to a tenth aspect of the invention, a printing device controlprogram to control a printing device that prints an image on a mediumused for printing using a print head having nozzles capable of formingdots causes a computer to perform processing as follows: acquiring imagedata having an M-value (M≧3) pixel value; selecting specific pixel datain the image data; performing N-value conversion processing that isprocessing to covert the M-value (M≧3) pixel value indicated by theselected pixel data to an N (M>N≧2) value; diffusing the error to pixeldata that has not been subjected to the N-value conversion processing inthe image data according to the error diffusion matrix by using adifference between a value of the selected pixel data and a value of theselected pixel data after the N-value conversion processing as theerror, thereby updating pixel values in the image data; generating printdata that defines information about a dot formation content of thenozzles corresponding to the image data after the N-value conversionprocessing; and printing the image on the medium using the print headaccording to the print data. When diffusing the error, a specific errordiffusion matrix is selected for each selected pixel data from an errordiffusion matrix storage portion having stored plural kinds of errordiffusion matrices having different diffusion ratios of the errorassigned to pixel data as diffusion targets according to nozzleinformation indicating a characteristic of each nozzle, and the error isdiffused to the pixel data that has not been subjected to the N-valueconversion processing according to the selected error diffusion matrix.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the printingdevice of the first aspect.

Most of the commercially available printing devices, such as an ink-jetprinter, are provided with a computer system including a centralprocessing unit (CPU), storage devices (RAM and ROM), and aninput/output device. The respective portions can be therefore achievedby software using the computer system. The respective portions can bethus achieved more economically and readily than in a case where therespective portions are achieved by forming exclusive-use hardware.

Further, the functions can be upgraded by changing and modifying thefunctions by re-writing the program partially.

According to an eleventh aspect of the invention, in the printing devicecontrol program of the tenth aspect, the nozzle information includesejection failure information indicating absence or presence of an inkejection failure for each nozzle.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the printingdevice of the second aspect.

According to a twelfth aspect of the invention, in the printing devicecontrol program of the tenth aspect, the nozzle information includesinformation about a quantity of position displacement between an actualforming position of a dot by each nozzle and an ideal forming positionof the dot.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the printingdevice of the third aspect.

According to a thirteenth aspect of the invention, in the printingdevice control program of any one of the tenth through twelfth aspects,the error diffusion matrix includes a diagonally weighted errordiffusion matrix that is an error diffusion matrix in which thediffusion ratio of the error assigned to the pixel data corresponding topixels as diffusion targets positioned in a diagonal direction, which isa direction other than a pixel selecting direction and a directionperpendicular to the pixel selecting direction, is made larger than thediffusion ratio assigned to the pixel data as diffusion targetspositioned in a direction other than the diagonal direction.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the printingdevice of the fourth aspect.

According to a fourteenth aspect of the invention, in the printingdevice control program of the thirteenth aspect, when diffusing theerror of pixel data corresponding to at least one of a nozzle having aquantity of the position displacement equal to or larger than a specificquantity and a nozzle in a neighborhood, the error of the pixel data isdiffused to the pixel data that has not been subjected to the N-valueconversion processing using the diagonally weighted error diffusionmatrix.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the printingdevice of the fifth aspect.

According to a fifteenth aspect of the invention, in the printing devicecontrol program of the thirteenth or fourteenth aspect, when diffusingthe error of pixel data corresponding to at least one of a nozzle havingan ink ejection failure and a nozzle in a neighborhood, the error of thepixel data is diffused to the pixel data that has not been subjected tothe N-value conversion processing using the diagonally weighted errordiffusion matrix.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the printingdevice of the sixth aspect.

According to a sixteenth aspect of the invention, in the printing devicecontrol program of any one of the twelfth through fourteenth aspects,when performing the N-value conversion processing, the pixel value ofthe pixel data corresponding to a nozzle having an ink ejection failureis converted to a value close to one of a lowest density value and alowest luminance value, and when diffusing the error, the error isdiffused to the pixel data that has not been subjected to the N-valueconversion processing in a neighborhood of the pixel data correspondingto the nozzle having an ink ejection failure using the error diffusionmatrix by using a difference between the pixel value of the pixel databefore the conversion and the pixel value of the pixel data after theconversion as the error.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the printingdevice of the seventh aspect.

According to a seventeenth aspect of the invention, a computer-readablerecording medium having stored a printing device control program hasstored the printing device control program of any one of the tenththrough sixteenth aspects.

Hence, not only can the same functions and advantages as those achievedby the printing device control program of any one of the tenth throughsixteenth aspects be achieved, but also the printing program can bereadily received via a recording medium, such as a CD-ROM, a DVD-ROM,and an MO.

According to an eighteenth aspect of the invention, a printing devicecontrol method for controlling a printing device that prints an image ona medium used for printing using a print head having nozzles capable offorming dots includes: acquiring image data having an M-value (M≧3)pixel value; selecting specific pixel data in the image data; performingN-value conversion processing that is processing to covert the M-value(M≧3) pixel value indicated by the selected pixel data to an N (M>N≧2)value; diffusing the error to pixel data that has not been subjected tothe N-value conversion processing in the image data according to theerror diffusion matrix by using a difference between a value of theselected pixel data and a value of the selected pixel data after theN-value conversion processing as the error, thereby updating pixelvalues in the image data; generating print data that defines informationabout a dot formation content of the nozzles corresponding to the imagedata after the N-value conversion processing; and printing the image onthe medium using the print head according to the print data. Whendiffusing the error, a specific error diffusion matrix is selected foreach selected pixel data from an error diffusion matrix storage portionhaving stored plural kinds of error diffusion matrices having differentdiffusion ratios of the error assigned to pixel data as diffusiontargets according to nozzle information indicating a characteristic ofeach nozzle, and the error is diffused to the pixel data that has notbeen subjected to the N-value conversion processing according to theselected error diffusion matrix.

The same function and advantage achieved by the printing device of thefirst aspect can be thus achieved.

According to a nineteenth aspect of the invention, in the printingdevice control method of the eighteenth aspect, the nozzle informationincludes ejection failure information indicating absence or presence ofan ink ejection failure for each nozzle.

The same function and advantage achieved by the printing device of thesecond aspect can be thus achieved.

According to a twentieth aspect of the invention, in the printing devicecontrol method of the eighteenth aspect, the nozzle information includesinformation about a quantity of position displacement between an actualforming position of a dot by each nozzle and an ideal forming positionof the dot.

The same function and advantage achieved by the printing device of thethird aspect can be thus achieved.

According to the twenty-first aspect of the invention, in the printingdevice control method of any one of the eighteenth through twentiethaspects, the error diffusion matrix includes a diagonally weighted errordiffusion matrix that is an error diffusion matrix in which thediffusion ratio of the error assigned to the pixel data corresponding topixels as diffusion targets positioned in a diagonal direction, which isa direction other than a pixel selecting direction and a directionperpendicular to the pixel selecting direction, is made larger than thediffusion ratio assigned to the pixel data as diffusion targetspositioned in a direction other than the diagonal direction.

The same function and advantage achieved by the printing device of thefourth aspect can be thus achieved.

According to a twenty-second aspect of the invention, in the printingdevice control method of the twenty-first aspect, when diffusing theerror of pixel data corresponding to at least one of a nozzle having aquantity of the, position displacement equal to or larger than aspecific quantity and a nozzle in a neighborhood, the error of the pixeldata is diffused to the pixel data that has not been subjected to theN-value conversion processing using the diagonally weighted errordiffusion matrix.

The same function and advantage achieved by the printing device of thefifth aspect can be thus achieved.

According to a twenty-third aspect of the invention, in the printingdevice control method of the twenty-first or twenty-second aspect, whendiffusing the error of pixel data corresponding to at least one of anozzle having an ink ejection failure and a nozzle in a neighborhood,the error of the pixel data is diffused to the pixel data that has notbeen subjected to the N-value conversion processing using the diagonallyweighted error diffusion matrix.

The same function and advantage achieved by the printing device of thesixth aspect can be thus achieved.

According to a twenty-fourth aspect of the invention, in the printingdevice control method of any one of the twentieth through twenty-thirdaspects, when performing the N-value conversion processing, the pixelvalue of the pixel data corresponding to a nozzle having an ink ejectionfailure is converted to a value close to one of a lowest density valueand a lowest luminance value, and when diffusing the error, the error isdiffused to the pixel data that has not been subjected to the N-valueconversion processing in a neighborhood of the pixel data correspondingto the nozzle having an ink ejection failure using the error diffusionmatrix by using a difference between the pixel value of the pixel databefore the conversion and the pixel value of the pixel data after theconversion as the error.

The same function and advantage achieved by the printing device of theseventh aspect can be thus achieved.

According to a twenty-fifth aspect of the invention, a print datageneration device that generates print data used in a printing devicethat prints an image on a medium used for printing using a print headhaving nozzles capable of forming dots includes: an image dataacquisition portion that acquires image data having an M-value (M≧3)pixel value; a nozzle information storage portion that stores nozzleinformation indicating a characteristic of each nozzle; an errordiffusion matrix storage portion that stores plural kinds of errordiffusion matrices having different diffusion ratios of an errorassigned to pixel data as diffusion targets; a pixel data selectingportion that selects specific pixel data in the image data; an N-valueconversion processing portion that performs N-value conversionprocessing that is processing to covert the M-value (M≧3) pixel valueindicated by the selected pixel data to an N (M>N≧2) value; an errordiffusion portion that diffuses the error to pixel data that has notbeen subjected to the N-value conversion processing in the image dataaccording to the error diffusion matrix by using a difference between avalue of the selected pixel data and a value of the selected pixel dataafter the N-value conversion processing as the error, thereby updatingpixel values in the image data; and a print data generation portion thatgenerates print data that defines information about a dot formationcontent of the nozzles corresponding to the image data after the N-valueconversion processing. The error diffusion portion selects a specificerror diffusion matrix for each selected pixel data from the errordiffusion matrix storage portion according to the nozzle information,and diffuses the error to the pixel data that has not been subjected tothe N-value conversion processing according to the selected errordiffusion matrix.

In other words, according to the twenty-fifth aspect, a printing portionto actually execute printing as the one in the printing device isomitted, and instead, print data that matches with the characteristic ofthe print head is generated according to the original M-value imagedata.

Hence, not only can the same function and advantage as the printingdevice of the first aspect be achieved, but also printing can beexecuted on a printing device by merely transmitting the print datagenerated according to the twenty-fifth aspect to the printing device.Moreover, when configured in this manner, an existing ink-jet printingdevice can be used intact without having to prepare a special printingdevice.

In addition, because a general-purpose information processing device,such as a personal computer, can be used, an existing printing systemincluding a printing instruction device, such as a personal computer,and an ink-jet printer, can be used intact.

According to a twenty-sixth aspect of the invention, in the print datageneration device of the twenty-fifth aspect, the nozzle informationincludes ejection failure information indicating absence or presence ofan ink ejection failure for each nozzle.

The same function and advantage achieved by the printing device of thesecond aspect can be thus achieved.

According to a twenty-seventh aspect of the invention, in the print datageneration device of the twenty-fifth aspect, the nozzle informationincludes information about a quantity of position displacement betweenan actual forming position of a dot by each nozzle and an ideal formingposition of the dot.

The same function and advantage achieved by the printing device of thethird aspect can be thus achieved.

According to a twenty-eighth aspect of the invention, in the print datageneration device of any one of the twenty-fifth through twenty-seventhaspects, the error diffusion matrix includes a diagonally weighted errordiffusion matrix that is an error diffusion matrix in which thediffusion ratio of the error assigned to the pixel data corresponding topixels as diffusion targets positioned in a diagonal direction, which isa direction other than a pixel selecting direction and a directionperpendicular to the pixel selecting direction, is made larger than thediffusion ratio assigned to the pixel data as diffusion targetspositioned in a direction other than the diagonal direction.

The same function and advantage achieved by the printing device of thefourth aspect can be thus achieved.

According to a twenty-ninth aspect of the invention, in the print datageneration device of the twenty-eighth aspect, in the error diffusionprocessing for the error of pixel data corresponding to at least one ofa nozzle having a quantity of the position displacement equal to orlarger than a specific quantity and a nozzle in a neighborhood, theerror diffusion portion diffuses the error of the pixel data to thepixel data that has not been subjected to the N-value conversionprocessing using the diagonally weighted error diffusion matrix.

The same function and advantage achieved by the printing device of thefifth aspect can be thus achieved.

According to a thirtieth aspect of the invention, in the print datageneration device of the twenty-eighth or twenty-ninth aspect, in theerror diffusion processing for the error of pixel data corresponding toat least one of a nozzle having an ink ejection failure and a nozzle ina neighborhood, the error diffusion portion diffuses the error of thepixel data to the pixel data that has not been subjected to the N-valueconversion processing using the diagonally weighted error diffusionmatrix.

The same function and advantage achieved by the printing device of thesixth aspect can be thus achieved.

According to a thirty-first aspect of the invention, in the print datageneration device of any one of the twenty-seventh through twenty-ninthaspects, the N-value conversion processing portion converts the pixelvalue of the pixel data corresponding to a nozzle having an ink ejectionfailure to a value close to one of a lowest density value and a lowestluminance value, and the error diffusion portion diffuses the error tothe pixel data that has not been subjected to the N-value conversionprocessing in a neighborhood of the pixel data corresponding to thenozzle having an ink ejection failure using the error diffusion matrixby using a difference between the pixel value of the pixel data beforethe conversion and the pixel value of the pixel data after theconversion as the error.

The same function and advantage achieved by the printing device of theseventh aspect can be thus achieved.

According to a thirty-second aspect of the invention, a print datageneration program to generate print data used in a printing device thatprints an image on a medium used for printing using a print head havingnozzles capable of forming dots causes a computer to perform processingas follows: acquiring image data having an M-value (M≧3) pixel value;selecting specific pixel data in the image data; performing N-valueconversion processing that is processing to covert the M-value (M≧3)pixel value indicated by the selected pixel data to an N (M>N≧2) value;diffusing the error to pixel data that has not been subjected to theN-value conversion processing in the image data according to the errordiffusion matrix by using a difference between a value of the selectedpixel data and a value of the selected pixel data after the N-valueconversion processing as the error, thereby updating pixel values in theimage data; and generating print data that defines information about adot formation content of the nozzles corresponding to the image dataafter the N-value conversion processing. When diffusing the error, aspecific error diffusion matrix is selected for each selected pixel datafrom an error diffusion matrix storage portion having stored pluralkinds of error diffusion matrices having different diffusion ratios ofthe error assigned to pixel data as diffusion targets according tonozzle information indicating a characteristic of each nozzle, and theerror is diffused to the pixel data that has not been subjected to theN-value conversion processing according to the selected error diffusionmatrix.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the print datageneration device of the twenty-fifth aspect.

According to a thirty-third aspect of the invention, in the print datageneration program of the thirty-second aspect, the nozzle informationincludes ejection failure information indicating absence or presence ofan ink ejection failure for each nozzle.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the print datageneration device of the twenty-fifth aspect.

According to a thirty-fourth aspect of the invention, in the print datageneration program of the thirty-second aspect, the nozzle informationincludes information about a quantity of position displacement betweenan actual forming position of a dot by each nozzle and an ideal formingposition of the dot.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the print datageneration device of the twenty-sixth aspect.

According to a thirty-fifth aspect of the invention, in the print datageneration program of any one of the thirty-second through thirty-fourthaspects, the error diffusion matrix includes a diagonally weighted errordiffusion matrix that is an error diffusion matrix in which thediffusion ratio of the error assigned to the pixel data corresponding topixels as diffusion targets positioned in a diagonal direction, which isa direction other than a pixel selecting direction and a directionperpendicular to the pixel selecting direction, is made larger than thediffusion ratio assigned to the pixel data as diffusion targetspositioned in a direction other than the diagonal direction.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the print datageneration device of the twenty-eighth aspect.

According to a thirty-sixth aspect of the invention, in the print datageneration program of the thirty-fifth aspect, when diffusing the errorof pixel data corresponding to at least one of a nozzle having aquantity of the position displacement equal to or larger than a specificquantity and a nozzle in a neighborhood, the error of the pixel data isdiffused to the pixel data that has not been subjected to the N-valueconversion processing using the diagonally weighted error diffusionmatrix.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the print datageneration device of the twenty-ninth aspect.

According to a thirty-seventh aspect of the invention, in the print datageneration program of the thirty-fifth or thirty-sixth aspect, whendiffusing the error of pixel data corresponding to at least one of anozzle having an ink ejection failure and a nozzle in a neighborhood,the error of the pixel data is diffused to the pixel data that has notbeen subjected to the N-value conversion processing using the diagonallyweighted error diffusion matrix.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the print datageneration device of the thirtieth aspect.

According to a thirty-eighth aspect of the invention, in the print datageneration program of any one of the thirty-fourth through thirty-sixthaspects, when performing the N-value conversion processing, the pixelvalue of the pixel data corresponding to a nozzle having an ink ejectionfailure is converted to a value close to one of a lowest density valueand a lowest luminance value, and when diffusing the error, the error isdiffused to the pixel data that has not been subjected to the N-valueconversion processing in a neighborhood of the pixel data correspondingto the nozzle having an ink ejection error using the error diffusionmatrix by using a difference between the pixel value of the pixel databefore the conversion and the pixel value of the pixel data after theconversion as the error.

When configured in this manner, as the computer reads the program andthe computer starts the processing according the read program, there canbe achieved the same function and advantage achieved by the print datageneration device of the thirty-first aspect.

According to a thirty-ninth aspect of the invention, a computer-readablerecording medium having recorded a print data generation program hasrecorded the print data generation program of any one of thethirty-second through thirty-eighth aspects.

Hence, not only can the same functions and advantages as those achievedby the print data generation program of any one of the thirty-secondthrough thirty-eighth aspects be achieved, but also the printing programcan be readily received via a recording medium, such as a CD-ROM, aDVD-ROM, and an FD (Flexible Disc).

According to a fortieth aspect of the invention, a print data generationmethod for generating print data used in a printing device that printsan image on a medium used for printing using a print head having nozzlescapable of forming dots includes: acquiring image data having an M-value(M≧3) pixel value; selecting specific pixel data in the image data;performing N-value -conversion processing that is processing to covertthe M-value (M≧3) pixel value indicated by the selected pixel data to anN (M>N≧2) value; diffusing the error to pixel data that has not beensubjected to the N-value conversion processing in the image dataaccording to the error diffusion matrix by using a difference between avalue of the selected pixel data and a value of the selected pixel dataafter the N-value conversion processing as the error, thereby updatingpixel values in the image data; and generating print data that definesinformation about a dot formation content of the nozzles correspondingto the image data after the N-value conversion processing. Whendiffusing the error, a specific error diffusion matrix is selected foreach selected pixel data from an error diffusion matrix storage portionhaving stored plural kinds of error diffusion matrices having differentdiffusion ratios of the error assigned to pixel data as diffusiontargets according to nozzle information indicating a characteristic ofeach nozzle, and the error is diffused to the pixel data that has notbeen subjected to the N-value conversion processing according to theselected error diffusion matrix.

The same function and advantage as the print data generation device ofthe twenty-fifth aspect can be thus achieved.

According to a forty-first aspect of the invention, in the print datageneration method of the fortieth aspect, the nozzle informationincludes ejection failure information indicating absence or presence ofan ink ejection failure for each nozzle.

The same function and advantage as the print data generation device ofthe twenty-sixth aspect can be thus achieved.

According to a forty-second aspect of the invention, in the print datageneration method of the fortieth aspect, the nozzle informationincludes information about a quantity of position displacement betweenan actual forming position of a dot by each nozzle and an ideal formingposition of the dot.

The same function and advantage as the print data generation device ofthe twenty-seventh aspect can be thus achieved.

According to a forty-third aspect of the invention, in the print datageneration method of any one of the fortieth through forty-secondaspects, the error diffusion matrix includes a diagonally weighted errordiffusion matrix that is an error diffusion matrix in which thediffusion ratio of the error assigned to the pixel data corresponding topixels as diffusion targets positioned in a diagonal direction, which isa direction other than a pixel selecting direction and a directionperpendicular to the pixel selecting direction, is made larger than thediffusion ratio assigned to the pixel data as diffusion targetspositioned in a direction other than the diagonal direction.

The same function and advantage as the print data generation device ofthe twenty-eighth aspect can be thus achieved.

According to a forty-fourth aspect of the invention, in the print datageneration method of the forty-third aspect, when diffusing the error ofpixel data corresponding to at least one of a nozzle having a quantityof the position displacement equal to or larger than a specific quantityand a nozzle in a neighborhood, the error of the pixel data is diffusedto the pixel data that has not been subjected to the N-value conversionprocessing using the diagonally weighted error diffusion matrix.

The same function and advantage as the print data generation device ofthe twenty-ninth aspect can be thus achieved.

According to a forty-fifth aspect of the invention, in the print datageneration method of the forty-third or forty-fourth aspect, whendiffusing the error of pixel data corresponding to at least one of anozzle having an ink ejection failure and a nozzle in a neighborhood,the error of the pixel data is diffused to the pixel data that has notbeen subjected to the N-value conversion processing using the diagonallyweighted error diffusion matrix.

The same function and advantage as the print data generation device ofthe thirtieth aspect can be thus achieved.

According to a forty-sixth aspect of the invention, in the print datageneration method of any one of the forty-second through forty-fourthaspects, when performing the N-value conversion processing, the pixelvalue of the pixel data corresponding to a nozzle having an ink ejectionfailure is converted to a value close to one of a lowest density valueand a lowest luminance value, and when diffusing the error, the error isdiffused to the pixel data that has not been subjected to the N-valueconversion processing in a neighborhood of the pixel data correspondingto the nozzle having an ink ejection failure using the error diffusionmatrix by using a difference between the pixel value of the pixel databefore the conversion and the pixel value of the pixel data after theconversion as the error.

The same function and advantage as the print data generation device ofthe thirty-first aspect can be thus achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a block diagram showing the configuration of a printing deviceof the invention.

FIG. 2 is a view showing the hardware configuration of a computersystem.

FIG. 3 is a partial enlarged bottom view showing the structure of aprint head of the invention.

FIG. 4 is a partial enlarged side elevation of the print head.

FIG. 5 is a flowchart detailing printing processing by the printingdevice.

FIG. 6 is a flowchart detailing print data generation processing by theprinting device according to a first embodiment of the invention.

FIG. 7 is a view showing one example of a dot pattern formed by a blacknozzle module alone having no failing nozzle that causes the flightdeviation.

FIG. 8 is a view showing one example of a dot pattern formed by theblack nozzle module having a nozzle that causes the flight deviationphenomenon.

FIG. 9 is a view showing one example of a dot pattern formed by theblack nozzle module having a nozzle that has an ink ejection failure(herein, non-ejection).

FIG. 10 is a view showing one example of information about an N valueand information about the threshold for each N value regarding a dotsize.

FIG. 11A is a view showing absolute ejection accuracy information(information about a quantity of flight deviation) for each nozzle.

FIG. 11B is a view showing relative ejection accuracy information foreach nozzle.

FIG. 12 is a view used to describe the reason why the relative ejectionaccuracy information is necessary.

FIG. 13 is a view showing the absence or presence of an ejection failure(herein, non-ejection) for each nozzle.

FIG. 14A is a view showing one example of the configuration of an errordiffusion matrix.

FIG. 14B is a view showing diffusion ratios set in respective errordiffusion matrices and kinds of error diffusion matrices stored in anerror diffusion matrix storage portion.

FIG. 15 is a flowchart detailing the print data generation processing bythe printing device.

FIG. 16 is a gradation image used in one example.

FIG. 17 is a view showing a normal error diffusion matrix used in theexample.

FIG. 18 is a view showing the result when the gradation image of FIG. 16is subjected to quaternary conversion using the error diffusion matrixshown in FIG. 17.

FIG. 19 is a view showing the result when the gradation image of FIG. 16is subjected to quaternary conversion through an error diffusionprocessing technique of the invention.

FIG. 20A through FIG. 20C are views used to describe a differencebetween the printing method of a multi-pass ink-jet printer and theprinting method of a line-head ink-jet printer.

FIG. 21 is a conceptual view showing another example of the structure ofthe print head.

FIG. 22A through FIG. 22C are views used to describe weight vectors.

DESCRIPTION OF EXEMPLARY EMBODIMENTS First Embodiment

Hereinafter, a first embodiment of the invention will be described withreference to the accompanying drawings. FIG. 1 through FIG. 14 are viewsshowing the first embodiment of a printing device, a printing devicecontrol program, a printing device control method, a print datageneration device, a print data generation program, and a print datageneration method of the invention.

Firstly, the configuration of a printing device 100 of the inventionwill be described with reference to FIG. 1. FIG. 1 is a block diagramshowing the configuration of the printing device 100 of the invention.

The printing device 100 is a line-head printing device. As is shown inFIG. 1, it comprises an image data acquisition portion 10 that acquiresM-value (M≧3) image data from an external device or a storage medium, aprint data generation portion 11 that generates print data used for aprinting portion 12 described below to print an image of the image dataon a print medium (for example, a print sheet) by performing N-value(M>N≧2) conversion processing on the image data acquired from the imagedata acquisition portion 10, and the printing portion 12 that prints theimage of the image data onto the print medium according to the printdata by the ink-jet method.

The image data acquisition portion 10 is furnished with a function ofacquiring M-value (herein, 256≧M≧3) image data for which the grayscale(luminance value) of respective colors (R, G, and B) for each pixel isrepresented by 8 bits (0 to 255). The image data acquisition portion 10acquires such image data from: an external device via a network, such asthe LAN and the WAN; a recording medium, such as a CD-ROM and a DVD-ROM,via an unillustrated driving device, such as a CD drive and a DVD drive,equipped to the printing device 100; or a storage device 70 equipped tothe printing device 100 described below. Further, the image dataacquisition portion 10 is able to exert a function of subjecting theM-value RGB data to color conversion processing to be converted toM-value CMYK (in the case of four colors) data corresponding torespective inks in a print head 200.

The print data generation portion 11 comprises an N-value conversionportion 11 a, an N-value conversion information storage portion 11 b, anerror diffusion portion 11 c, a nozzle information storage portion 11 d,an error diffusion matrix storage portion 11 e, and a print datageneration memory 11 f.

The N-value conversion portion 11 a stores the image data that has beensubjected to CMYK color conversion into the print data generation memory11 f. Also, it selects specific pixel data from the image data andconverts the specific pixel data thus selected (hereinafter, referred toas the selected pixel data) to an N value according to an N-valueconversion threshold, a dot number corresponding to each dot formationsize, and a pixel value (for example, a luminance value) after theN-value conversion corresponding to each dot number, all of which arecontained in the N-value conversion information read from the N-valueconversion information storage portion 11 b. Further, it calculates adifference between the pixel value before the N-value conversion and thepixel value after the N-value conversion of the selected pixel data, andtransmits the difference as an error to the error diffusion portion 11 ctogether with the information about the selected pixel data. The valueafter the N-value conversion takes either the dot number correspondingto the original pixel value or a numerical value “0” indicating not toform a dot. The N-value conversion portion 11 a converts the dataindicating the value after the N-value conversion to data that theprinting portion 12 can interpret, and stores the converted data in theprint data generation memory 11 f by writing the converted data over theoriginal pixel data that has been stored before (the converted data maybe stored in another region instead of being written over).

The N-value conversion referred to herein means the processing toconvert M-value (M≧3) image data (having M kinds of pixel values (pixeldata)) to N-value (M>N≧2) data (having N kinds of numerical values). Forexample, in the case of binarization, a pixel value to be converted iscompared with the threshold, and this pixel value is converted to one oftwo kinds of pre-set numerical values as follows: when the pixel valueto be converted is equal to or greater than the threshold, this pixelvalue is converted to a numerical value “1”, and when the pixel value tobe converted is smaller than the threshold, this pixel value isconverted to a numerical value “0”. Hence, in the case of the N-valueconversion, the M-value pixel value is compared with N kinds ofthreshold values, and it is converted to any one numerical value amongpre-set N kinds of numerical values in response to the comparisonresult.

As has been described, the N-value conversion information storageportion 11 b stores N-value conversion information made of an N-valueconversion threshold corresponding to a dot formation size of eachnozzle, the dot number corresponding to each dot formation size, a pixelvalue (for example, a luminance value) after the N-value conversioncorresponding to each dot number, etc.

The error diffusion portion 11 c reads out nozzle informationcorresponding to the selected pixel data from the nozzle informationstorage portion 11 d according to the information and the error of theselected pixel data acquired from the N-value conversion portion 11 a,and judges whether the nozzle corresponding to the selected pixel datais a failing nozzle according to the nozzle information. Upon judgingthat the nozzle in question is a failing nozzle, it selectively readsout a special error diffusion matrix for avoiding the banding phenomenoncaused by this failing nozzle from the error diffusion matrix storageportion 11 e. Meanwhile, upon judging that the nozzle in question is anormal nozzle having no trouble, it reads out a common error diffusionmatrix prepared for a normal nozzle (hereinafter, referred to as thenormal error diffusion matrix). By using the error diffusion matrix thusread, the error diffusion portion 11 c diffuses (distributes) the errorto the pixel data that has not been subjected to the N-value conversionprocessing in the neighborhood of the pixel corresponding to theselected pixel data in the image data stored in the print datageneration memory 11 f. The image data stored in the print datageneration memory 11 f is updated by diffusing the error to the pixelvalues as diffusion targets in this manner each time the selected pixeldata is converted to an N value.

The error diffusion processing referred to herein is to diffuse an erroron the same principle underlying the error diffusion method in therelated art. For example, in the case of binarization processing where“128” is given as the threshold and N-value image data is converted to“0” when the pixel value is smaller than “128” and to “255” when thepixel value is equal to or greater than “128”, assume that the pixelvalue of a selected pixel is “101”, then “101” is converted to “0”. Adifference, “101”, between “0” after the conversion and “101” before theconversion is deemed as an error and diffused to more than oneoutstanding pixel in the neighborhood according to a predetermineddiffusion method. For example, a pixel immediately to the right of theselected pixel (for example, having the pixel value of “101”) isconverted to “0” as with the selected pixel by the normal binarizationprocessing because it is also smaller than the threshold. However, byreceiving an error of the selected pixel, for example, “27”, the pixelvalue is increased to “128” and becomes equal to the threshold “128”.The pixel value of this pixel is therefore converted to “256”.

Also, the failing nozzle referred to herein includes all nozzlesinvolved in the occurrence of the banding phenomenon, such as a nozzlehaving an ink ejection failure, for example, a nozzle incapable ofejecting ink or a nozzle that ejects an abnormal quantity of ink, aswell as a nozzle that causes the flight deviation phenomenon due to thedisplacement of the dot forming position from the ideal position.

The nozzle information storage portion 11 d stores nozzle informationmade of information indicating the correspondence between each nozzle Nin the print head 200 provided to the printing portion 12 and respectivepixel data in the image data, information indicating the absence orpresence of an ink ejection failure for each nozzle N, and informationindicating the characteristic of each nozzle N, such as informationindicating a quantity of flight deviation of each nozzle N. By checkingeach nozzle N in the print head 200 as to the occurrence of the flightdeviation phenomenon and the absence or presence of an ejection failureon the basis of the nozzle information, it is possible to judge whethereach nozzle N is a failing nozzle. Moreover, it is possible to specify afailing nozzle N, for example, in which ordinal position on the printhead 200 the failing nozzle N is present and to which pixel data in theimage data the failing nozzle corresponds.

The error diffusion matrix storage portion 11 e stores information aboutthe pixel data as diffusion targets, and matrix data that definesdiffusion ratios of the error assigned to respective pixel data as thediffusion targets in the form of plural kinds of error diffusionmatrices having different diffusion ratios in this embodiment.

In this embodiment, plural kinds of diagonally weighted error diffusionmatrices are stored as plural kinds of error diffusion matrices, inwhich the diffusion ratio is increased for pixel data corresponding topixels as diffusion targets present in a diagonal direction with respectto the pixel corresponding to the selected pixel data in comparison withother pixel data as diffusion targets for use in the error diffusionprocessing of the selected pixel data involved in the bandingphenomenon. Besides these matrices, plural kinds of normal errordiffusion matrices for pixel data that is not involved in the bandingphenomenon are also stored.

The print data generation memory 11 f stores image data having CMYKcolor information acquired from the image data acquisition portion 10.The print data generation memory 11 f also stores the selected pixeldata converted to an N value by writing the converted selected pixeldata over the original pixel data according to an instruction from theN-value conversion portion 11 a, and also stores pixel data as diffusiontargets to which the error of the selected pixel data has been diffusedby writing such pixel data over the original pixel data according to aninstruction from the error diffusion portion 11 c. By updating theoriginal pixel data by performing the N-value conversion processing andthe error diffusion processing sequentially in this manner, all thepixel data in the image data acquired from the image data acquisitionportion 10 is consequently converted to N values. Data that the printingportion 12 can interpret, that is, the print data, is thus generated onthe memory. The print data is the data for printing used in the printingportion 12 of the ink jet method described below, that is, the dataindicating whether a dot of a specific color and a specific size isformed or no dot is formed for each pixel data in the image data.

FIG. 3 is a partial enlarged bottom view showing the structure of theprint head 200 of the invention. FIG. 4 is a partial enlarged sideelevation of the print head 200.

As is shown in FIG. 3, the print head 200 comprises four nozzle modules50, 52, 54, and 56. In the black nozzle module 50, plural nozzles N (18nozzles in the drawing) that eject black (K) ink alone are alignedlinearly in the nozzle alignment direction. In the yellow nozzle module52, plural nozzles N that eject yellow (Y) ink alone are alignedlinearly in the nozzle alignment direction. In the magenta nozzle module54, plural nozzles N that eject magenta (M) ink alone are alignedlinearly in the nozzle alignment direction. In the cyan nozzle module56, plural nozzles N that eject cyan (C) ink alone are aligned linearlyin the nozzle alignment direction. As is shown in FIG. 3, the nozzlemodules 50, 52, 54, and 56 are arrayed to form one unit in such a mannerthat the nozzles N labeled with the same numbers in these four nozzlemodules are aligned in a straight line along the printing direction (adirection perpendicular to the nozzle alignment direction). That is tosay, plural nozzles N that together form the respective nozzle modulesare aligned linearly in the nozzle alignment direction, and the nozzlesN labeled with the same numbers in the four nozzle modules are alignedlinearly in the printing direction.

Further, FIG. 4 shows a state where the sixth nozzle from the left, thenozzle N6, in the black nozzle module 50 among the four nozzle modules50, 52, 54, and 56 causes the flight deviation phenomenon, and ink isejected onto a print medium S in a diagonal direction from the nozzleN6, thereby forming a dot on the print medium S in the vicinity of a dotejected from a normal nozzle N7 adjacent to the nozzle N6 and formed onthe print medium S.

The printing portion 12 is an ink-jet printer that forms an image madeof a large number of dots on the print medium S by ejecting dot-shapedinks from the nozzle modules 50, 52, 54, and 56 provided to the printhead 200 while moving either the print medium S or the print head 200 orboth shown in FIG. 4. Besides the print head 200, the printing portion12 includes an unillustrated print head moving mechanism (in the case ofa multi-pass type) that moves the print head 200 over the print medium Sto reciprocate in the width direction, an unillustrated papertransportation mechanism that moves the print medium (sheet of paper) S,and an unillustrated printing control mechanism that controls ejectionof ink from the print head 200 according to the print data.

The printing device 100 achieves the respective functions, such as theimage data acquisition portion 10, the print data generation portion 11,and the printing portion 12 by software, and therefore is provided witha computer system that runs the software to control the hardware neededto achieve the respective functions. According to the hardwareconfiguration of the computer system as is shown in FIG. 2, a CPU(Central Processing Unit) 60 serving as an arithmetic and programcontrol that performs respective kinds of control and computationprocessing, a RAM (Random Access Memory) 62 that forms a main storagedevice (Main Storage), and a ROM (Read Only Memory) 64 serving as aread-only storage device are interconnected via various kinds ofinside/outside buses 68 comprising a PCI (Peripheral ComponentInterconnect) bus and an ISA (Industrial Standard Architecture) bus. Inaddition, an external storage device (Secondary Storage) 70 such as anHDD, an output device 72, such as a printing portion 20, a CRT, and anLCD monitor, and an input device 74, such as an operation panel, amouse, a keyboard, and a scanner, and a network cable L to enablecommunications with an unillustrated printing instruction device, areconnected to the buses 68 via an input/output interface (I/F) 66.

When the power supply is turned ON, a system program, such as BIOS,pre-installed in the ROM 64 down loads various kinds of exclusive-usecomputer programs pre-installed in the ROM 64 or various kinds ofexclusive-use computer programs installed in the storage device 70 via arecording medium, such as a CD-ROM, a DVD-ROM, and a flexible disc (FD),or via a communication network, such as the Internet, to the RAM 62.Respective functions as described above are achieved on the software asthe CPU 60 performs specific control and computation processing usingvarious resources according to instructions written in the programs downloaded to the RAM 62.

Further, the printing device 100 activates a specific programpre-installed in a specific region of the ROM 64 by the CPU 60, andexecutes the printing processing detailed in the flowchart of FIG. 5according to the program. As has been described, the print head 200 thatforms dots is normally able to form dots of more than one color, forexample, dots of four or six colors, simultaneously. However, for easeof description, descriptions will be given on the assumption that eachdot is formed by a (monochrome) print head 200 of one color (monochromeimage).

FIG. 5 is a flowchart detailing the printing processing by the printingdevice 100.

As is shown in FIG. 5, when the CPU 60 starts the printing processing,the printing device 100 proceeds to Step S100.

In Step S100, whether the image data acquisition portion 10 has receiveda printing instruction, either in the form of printing instructioninformation sent from a linked-external device via the network cable Lor in the form of printing instruction information inputted via theinput device 74, is judged. Upon judgment that the printing instructionhas been received (Yes), the flow proceeds to Step S102; otherwise (No),the judgment processing is repeated until the printing instruction isreceived.

When the flow proceeds to Step S102, the image data acquisition portion10 performs processing to acquire image data corresponding to theprinting instruction from the external device, a recording medium, suchas a CD-ROM and a DVD-ROM, or the storage device 70; such as an HDD, asdescribed above. Subsequently, whether the image data has been acquiredis judged. Upon judgment that the image data has been acquired (Yes),the flow proceeds to Step S104; otherwise (No), a response, “printing isimpossible”, is returned to the sender of the printing instruction. Theflow then returns to Step S100 by canceling the printing processing forthis printing instruction. The image data is the data made of pluralM-value pixel data arrayed in a matrix fashion. The row directioncoincides with the nozzle alignment direction of the print head 200, andthe column direction coincides with the printing direction of the printhead 200.

When the flow proceeds to Step S104, in a case where the M-value imagedata acquired in Step S102 is image data having color information otherthan CMYK, the image data acquisition portion 10 converts the image datato the image data having the color information of CMYK (hereinafter,referred to as CMYK image data), and transmits the CMYK image data tothe print data generation portion 11. The flow then proceeds to StepS106. In other words, the image data having the color information otherthan CMYK is subjected to CMYK conversion before it is transmitted,whereas the CMYK image data is transmitted intact.

In Step S106, when the M-value CMYK image data is acquired from theimage data acquisition portion 10, the print data generation portion 11generates the print data by performing the N-value conversion and theerror diffusion processing to the acquired image data. The flow thenproceeds to Step S108.

In Step S108, the print data generation portion 11 outputs the printdata generated in Step S106 to the printing portion 12. The flow thenproceeds to Step S110.

In Step S110, the printing portion 12 executes printing processingaccording to the print data from the print data generation portion 11.The flow then returns to Step S100.

The print data generation processing in Step S106 will now be describedin detail with reference to FIG. 6.

FIG. 6 is a flowchart detailing the print data generation processing bythe printing device 100.

The print data generation processing is the processing according towhich the following are performed. That is, the selected pixel data issubjected to the N-value conversion processing, and whether the nozzle Ncorresponding to the selected pixel data is a failing nozzle is judged.An appropriate error diffusion matrix for the pixel data correspondingto the failing nozzle is selected from the error diffusion matrixstorage portion lie according to the judgment result, and the errordiffusion processing is performed using the error diffusion matrix thusselected. The print data is then generated according to the image datathat has been subjected to the N-value conversion processing and theerror diffusion processing. When Step S106 is performed, the flowproceeds to Step S200 as is shown in FIG. 6.

In Step S200, the N-value conversion portion 11 a judges whether theM-value CMYK image data has been acquired from the image dataacquisition portion 10. Upon judgment that the CMYK image data has beenacquired (Yes), the acquired CMYK image is stored in the print datageneration memory 11 f. The flow then proceeds to Step S202. Otherwise(No), the judgment processing is repeated until the CMYK image data isacquired.

When the flow proceeds to Step S202, the N-value conversion portion 11 aacquires N-value conversion information by reading out the N-valueconversion information from the N-value conversion information storageportion 11 b, and storing the read N-value conversion information in aspecific region of the RAM 62. The flow then proceeds to Step S204.

In Step S204, the N-value conversion portion 11 a selects pixel datathat has not been subjected to the N-value conversion processing in theCMYK image data stored in the print data generation memory 11 f. Theflow then proceeds to Step S206.

In Step S206, the N-value conversion portion 11 a converts the selectedpixel data selected in Step S204 to an N value according to the N-valueconversion information acquired in Step S202, and converts the valueafter the N-value conversion to data indicating the dot size formed bythe nozzle for the value after the N-value conversion in the format thatthe printing portion 12 can interpret. The selected pixel data in theCMYK image data stored in the print data generation memory 11 f is thusupdated to the converted data. The flow then proceeds to Step S208.

In Step S208, the N-value conversion portion 11 a calculates adifference between the pixel value before the N-value conversion and thepixel value after the N-value conversion of the selected pixel dataaccording to the result of the N-value conversion in Step S206, andtransmits the calculation result as an error to the error diffusionportion 11 c together with information about the selected pixel data.The flow then proceeds to Step S210.

In Step S210, upon acquisition of the error calculated in Step S208 andthe information about the selected pixel data corresponding to the errorfrom the N-value conversion portion 11 a, the error diffusion portion 11c acquires nozzle information corresponding to the selected pixel databy reading out the nozzle information from the nozzle informationstorage portion 11 d and storing the read nozzle information in aspecific region of the RAM 62. The flow then proceeds to Step S212.Thereafter, by searching for the nozzle information corresponding to theselected pixel data through the nozzle information stored in the RAM 62first, it is possible to accelerate the acquisition processing of thepixel data using the same nozzle information.

In Step S212, the error diffusion portion 11 c judges whether the nozzleN corresponding to the selected pixel data is a failing nozzle on thebasis of the nozzle information acquired in Step S210. Upon judgmentthat the nozzle N is a failing nozzle (Yes), the flow proceeds to StepS214; otherwise (No), the flow proceeds to Step S220.

When the flow proceeds to Step S214, the error diffusion portion 11 cselectively reads out the diagonally weighted error diffusion matrixfrom the error diffusion matrix storage portion 11 e. The flow thenproceeds to Step S216. In this embodiment, as has been described, pluralkinds of matrix data having different diffusion ratios are stored in theerror diffusion matrix storage portion 11 e, and the diagonally weightederror diffusion matrix can be selected from these plural diagonallyweighted error diffusion matrices in various manners, for example, itmay be selected randomly or in a predetermined sequential order for eachselected pixel data, or it may be selected appropriately in response tothe condition of the failing nozzle corresponding to the selected pixel.

In Step S216, the error diffusion portion 11 c diffuses the error of theselected pixel data acquired in Step S210 according to the errordiffusion matrix selected in Step S214 or S220 to pixel data asdiffusion targets that has not been subjected to the N-value conversionprocessing in the image data stored in the print data generation memory11 f, and thereby updates the pixel data as the diffusion targets. Theflow then proceeds to Step S218.

In Step S218, the N-value conversion portion 11 a judges whether theN-value conversion processing has been completed for all the pixel data.Upon judgment that the processing has been completed (Yes), the sequenceof processing is ended and the device returns to the originalprocessing; otherwise (No), the flow returns to Step S204.

Meanwhile, when the flow proceeds to Step S220 upon judgment that thenozzle corresponding to the selected pixel data is not a failing nozzlein Step S212, the error diffusion portion 11 c selectively reads out thenormal error diffusion matrix from the error diffusion matrix storageportion 11 e. The flow then proceeds to Step S216. In this embodiment,as has been described, plural kinds of matrix data having differentdiffusion ratios are stored in the error diffusion matrix storageportion 11 e, and the normal error diffusion matrix can be selected fromthese error diffusion matrices in various manners, for example, the onecorresponding to each pixel data may be selected, or it may be selectedrandomly or in the predetermined sequential order for each selectedpixel data.

The operation of this embodiment will now be described with reference toFIG. 7 through FIG. 14.

FIG. 7 is a view showing one example of a dot pattern formed by theblack nozzle module 50 alone having no so-called failing nozzle. FIG. 8is a view showing one example of a dot pattern formed by the blacknozzle module 50 when the nozzle N6 causes the flight deviationphenomenon. FIG. 9 is a view showing one example of a dot pattern formedby the black nozzle module 50 when the nozzle N6 has an ink ejectionfailure (in the drawing, non-ejection). FIG. 10 is a view showing oneexample of information about an N value and information about thethreshold for each N value regarding the dot size. FIG. 11A is a viewshowing absolute ejection accuracy information (information about aquantity of flight deviation) for each nozzle. FIG. 11B is a viewshowing relative ejection accuracy information for each nozzle. FIG. 12is a view used to describe the reason why the relative ejection accuracyinformation is necessary. FIG. 13 is a view showing the absence orpresence of an ejection failure (in the drawing, non-ejection) for eachnozzle. FIG. 14A is a view showing one example of the configuration ofthe error diffusion matrix. FIG. 14B is a view showing the diffusionratios of the respective error diffusion matrices and the kinds of errordiffusion matrices stored in the error diffusion matrix storage portion11 e.

As is shown in FIG. 7, the banding phenomenon, such as a white streakand a dark streak, resulting from displacement of nozzle intervals willnot occur in the dot pattern formed by the black nozzle module 50 havingno failing nozzle.

On the contrary, as is shown in FIG. 8, in the printing result by theblack nozzle module 50 having a nozzle that causes the flight deviation,dots formed by the nozzle N6 is displaced by a distance a toward dotsformed by a normal nozzle N7 immediately to the right of the nozzle N6.Consequently, a white streak occurs between dots formed by the nozzle N6and dots formed by a nozzle N5 immediately to the left of the nozzle N6.

In a case where the nozzle modules 52, 54, and 56 corresponding to othercolors are used instead of the black nozzle module 50, as has beendescribed, dots formed by the nozzle N6 and dots formed by the nozzle N7immediately to the right come closer by the distance a as dots formed bythe nozzle N6 are displaced by the distance a due to the flightdeviation. Dots formed by these nozzles therefore become denser (dotsmay overlap), and this portion appears noticeably as a dark streak. Thequality of the printed matter is thus deteriorated considerably.

As is shown in FIG. 9, dots that should have been formed are not formeddue to an ink ejection failure (non-ejection) of the nozzle N6, and awhite streak occurs between dots formed by the nozzle N5 and dots formedby the nozzle N7.

The white streak becomes more noticeable in the printed matter printedat a uniform density, and in particular, in a combination of a whiteprint sheet and black ink having a significant difference in density.The quality of the printed matter is thus deteriorated extremely.

The printing device 100 according to this embodiment of the inventionbecomes able to produce print data to make a white streak or a darkstreak less noticeable by subjecting the pixel data corresponding to anozzle that causes the flight deviation or a nozzle having an ejectionfailure, that is, a failing nozzle to the error diffusion processingusing the diagonally weighted error diffusion matrix as described above.

Initially, in the printing device 100, upon receipt of the printinginstruction information from an external device (Step S100), the imagedata acquisition portion 10 acquires M-value image data corresponding tothe printing instruction information from the external device as thesender of the printing instruction information (Step S102). When theacquired image data has color information other than CMYK, the imagedata is converted to M-value CMYK image data, and the CMYK image data istransmitted to the print data generation portion 11 (Step S104).Meanwhile, upon acquisition of the CMYK image data from the image dataacquisition portion 10, the print data generation portion 11 starts toperform the print data generation processing (Step S106).

In the print data generation processing, the N-value conversion portion11 a first acquires the CMYK image data from the image data acquisitionportion 10, and then stores the CMYK image data thus acquired in theprint data generation memory 11 f (Step S200). Subsequently, the N-valueconversion portion 11 a reads out the N-value conversion informationfrom the N-value conversion information storage portion 11 b, and storesthe read N-value conversion information in a specific region of the RAM62 (Step S200).

Upon acquisition of the N-value conversion information, the N-valueconversion portion 11 a selects pixel data that has not been subjectedto the N-value conversion processing from the CMYK image data stored inthe print data generation memory 11 f (Step S204). The N-valueconversion portion 11 a then converts the value of the M-value selectedpixel data to an N value according to the acquired N-value conversioninformation. Also, it converts the value after the N-value conversion todata indicating the dot number of a dot formed by the nozzle for thisvalue after the N-value conversion in the format that the printingportion 12 can interpret. The selected pixel data in the CMYK image datastored in the print data generation memory 11 f is thus updated to theconverted data (Step S206).

In this embodiment, the N-value conversion is performed as follows. In acase where the original pixel value (luminance value (or density value))of the selected pixel data is 8 bits with 256 grayscale levels, as isshown in FIG. 10, when the original pixel value is smaller than “32”,then the pixel value is converted to “0”, and “7” is given as the Nvalue corresponding to the dot number. When the original pixel value isequal to “32” or greater and smaller than “64”, the pixel value isconverted to “36”, and “6” is given as the N value corresponding to thedot number. When the original pixel value is equal to or greater than“64” and smaller than “96”, the pixel value is converted to “73”, and“5” is given as the N value corresponding to the dot number. Likewise,when the original pixel value is equal to or greater than “96” andsmaller than “128”, the pixel value is converted to “109”, and “4” isgiven as the N value corresponding to the dot number. When the originalpixel value is equal to or greater than “128” and smaller than “159”,the pixel value is converted to “146”, and “3” is given as the N valuecorresponding to the dot number. When the original pixel value is equalto or greater than “159” and smaller than “191”, the pixel value isconverted to “182”, and “2” is given as the N value corresponding to thedot number. When the original pixel value is equal to or greater than“191” and smaller than “223”, the pixel value is converted to “219”, and“1” is given as the N value corresponding to the dot number. When theoriginal pixel value is equal to or greater than “223”, the pixel valueis converted to “255”, and “0” is given as the N value corresponding tothe dot number.

The example described above is a case where the luminance value isadopted as the pixel value, and when the density value is adopted as thepixel value, the value takes values in inverse order of the luminancevalues (for example, a value obtained by subtracting each luminancevalue from “255”).

Further, after the N-value conversion portion 11 a converted theselected pixel data to the N value, it calculates a difference betweenthe luminance value before the conversion and the luminance value afterthe conversion corresponding to the dot number of the selected pixeldata as an error, and transmits the error thus calculated to the errordiffusion portion 11 c together with the information about the selectedpixel data (Step S208).

Meanwhile, upon acquisition of the error and the information about theselected pixel data from the N-value conversion portion 11 a, the errordiffusion portion 11 c acquires nozzle information corresponding to theselected pixel data by reading out the nozzle information from thenozzle information storage portion 11 d according to the acquiredinformation about the selected pixel data and storing the nozzleinformation in a specific region of the RAM 62 (Step S210). The errordiffusion portion 11 c then judges whether the nozzle corresponding tothe selected pixel data is a failing nozzle according to the nozzleinformation thus acquired (Step S212).

In this embodiment, the judgment processing of a failing nozzle isperformed according to the absolute ejection accuracy information shownin FIG. 11A, the relative ejection accuracy information shown in FIG.11B, and ejection absence or presence information shown in FIG. 13.

For example, according to FIG. 11A, when a quantity of flight deviation(absolute ejection accuracy) of dots formed by the nozzle correspondingto the selected pixel data from the ideal position is less than ±4 μmand the nozzle N ejects ink, the occurrence of flight deviation is notjudged, and when a quantity of flight deviation is equal to or greaterthan ±4 μm, the occurrence of flight deviation is judged and the nozzleN corresponding to the selected pixel data is judged as being a failingnozzle.

However, even in a case where the absolute ejection accuracy is equal toor greater than ±4 μm, as is shown in FIG. 11B, the occurrence of flightdeviation is not judged when a difference (relative ejection accuracy)between the ideal position of the nozzle N corresponding to the selectedpixel data and the ideal position of a nozzle N+1 adjacent to the nozzleN is ±0 μm or less than a specific value.

The reason why the failing nozzle is judged using not only the absoluteejection accuracy but also the relative ejection accuracy as above isbecause as is shown in FIG. 12, for example, in a case where all ofthree sequentially aligned nozzles cause the flight deviation of thesame quantity in the same direction, when the error diffusion processingis performed on the pixel data corresponding to these nozzles with theuse of the special error diffusion matrix for avoiding banding (forexample, the diagonally weighted error diffusion matrix), there is apossibility that the image quality in the printing result isdeteriorated.

Further, according to the ejection absence or presence informationindicating whether the nozzle ejects ink as shown in FIG. 13, when theejection absence or presence information about the nozzle Ncorresponding to the selected pixel data shows “1”, the nozzle is judgedas being a failing nozzle incapable of ejecting ink. On the other hand,when the ejection absence or presence information shows “0” and thenozzle does not cause the flight deviation, the nozzle is judged asbeing a normal nozzle.

When the judgment processing as to whether the nozzle is a failingnozzle as described above ends, in a case where the nozzle Ncorresponding to the selected pixel data is judged as being a failingnozzle (branching to Yes in Step S212), the diagonally weighted errordiffusion matrix is selected from the error diffusion matrix storageportion 11 f (Step S214). On the other hand, when the nozzle Ncorresponding to the selected pixel data is judged as being a normalnozzle (branching to No in Step S212), a normal error diffusion matrixis selected from the error diffusion matrix storage portion 11 f (StepS220).

As is shown in FIG. 14A, the error diffusion matrix of this embodimentis a matrix having the diffusion content as follows. That is, let x1through x4 be error diffusion ratios assigned to four pixel data asdiffusion targets, then the error is diffused to the pixel data as adiffusion target at the lower left with respect to the selected pixeldata (pixel of interest in FIG. 14A) at the diffusion ratio, x1. Theerror is diffused to the pixel data as a diffusion target immediatelybelow at the diffusion ratio, x2. The error is diffused to the pixeldata as a diffusion target at the lower right at the diffusion ratio,x3. The error is diffused to the pixel data as a diffusion targetimmediately to the right at the diffusion ratio, x4.

The error diffusion matrix is selected as follows. That is, when thenozzle N corresponding to the selected pixel data is a failing nozzle,as is shown in FIG. 14B, the error diffusion matrix is selected fromdiagonally weighted error diffusion matrices identified with the matrixID, such as those identified with ID2, ID4, and ID8. On the other hand,when the nozzle N corresponding to the selected pixel data is a normalnozzle, the error diffusion matrix is selected from the normal errordiffusion matrices identified with the matrix ID, such as thoseidentified with ID1, ID3, and ID5 through ID7.

Numerical values corresponding to the diffusion ratios x1 through x4 inFIG. 14B represent the diffusion ratios, and descriptions will be givenusing the diagonally weighted error diffusion matrix identified with ID2as an example. As is shown in FIG. 14B, the diffusion ratios are set asfollows in the matrix identified with ID2: x1=1, x2=3, x3=7, and x4=5.When the error diffused to the respective pixel data as the diffusiontargets is calculated, these values are added up, 1+3+7+5=16, which isused as the denominator. Then, the error to be diffused (value to bedistributed) is calculated by multiplying the error by a numerator,which is one of the numerical values of x1 through x4. Hence, in theerror diffusion processing, as is shown in FIG. 14A, 1/16 of the erroris diffused to the pixel data corresponding to x1, and 3/16 of the erroris diffused to the pixel data corresponding to x2. Likewise, 7/16 of theerror is diffused to the pixel data corresponding to x3, and 5/16 of theerror is diffused to the pixel data corresponding to x4. In other words,it is understood that 7/16 of the error, which is larger than the errordiffused to any other pixel data, is diffused to the pixel data at thelower right with the use of the diagonally weighted error diffusionmatrix identified with ID2. Meanwhile, as is shown in FIG. 14B, thediffusion ratios are set as follows in the normal error diffusion matrixidentified with ID7: x1=3, x2=1, x3=5, and x4=7. When the normal errordiffusion matrix identified with ID7 is compared with the diagonallyweighted error diffusion matrix identified with ID2, it is understoodthat the error is diffused more in the diagonal direction (thediagonally right down direction) with the diagonally weighted errordiffusion matrix identified with ID2. The same applies to a comparisonbetween any other diagonally weighted error diffusion matrix and anyother normal error diffusion matrix. That is to say, the error diffusionratio is larger for the pixel data in the diagonal direction (pixel datacorresponding to x1 or x3 in FIG. 14A) in any diagonally weighted errordiffusion matrix than in any normal error diffusion matrix.

As has been described, as a manner in which the error diffusion matrixis selected from the error diffusion matrix storage portion 11 f, onediagonally weighted error diffusion matrix or one normal error diffusionmatrix is selected randomly for each selected pixel data among pluraldiagonally weighted error diffusion matrices and plural normal errordiffusion matrices pre-stored in the error diffusion matrix storageportion 11 f. Herein, the reason why the error diffusion matrix isselected randomly is because a better image quality can be obtained inthe printing result by diffusing the error with the use of plural kindsof randomly selected error diffusion matrices than by performing errordiffusion with the use of a fixed error diffusion matrix withoutvariation.

As has been described, once the error diffusion matrix is selected forthe selected pixel data, the errors calculated with the diffusion ratiosset in the selected error diffusion matrix as shown in FIG. 14B arediffused to the pixel data as diffusion targets at the lower left,immediately below, at the lower right, and immediately to the right withrespect to the selected pixel data (data of the pixel of interest inFIG. 14A) in the CMYK image data stored in the print data generationmemory 11 f as is shown in FIG. 14A. The CMYK image data stored in theprint data generation memory 11 f is thus updated.

Accordingly, the pixel value of pixel data that has not been subjectedto the N-value conversion processing in the neighborhood of the selectedpixel data is updated to the one that reflects the error caused by theN-value conversion as the result of the diffusion processing by theerror diffusion portion 11 c. Thereafter, the N-value conversion and theerror diffusion processing with the use of the error diffusion matrixselected according to the nozzle information are performed successivelyon the outstanding pixel data updated as described above.

The print data generation processing ends when the N-value conversionand the error diffusion processing have been performed on all the pixeldata in the CMYK image data stored in the print data generation memory11 f (branching to Yes in Step S218). In other words, each pixel valuein the CMYK image data after the N-value conversion and the errordiffusion processing stored in the print data generation memory 11 f isnow the data indicating the dot number corresponding to each dotformation size in the data format that the printing portion 12 caninterpret, and it is the print data that can drive the print head 200 inthe printing portion 12.

The print data generation portion 11 therefore outputs the print datathus generated to the printing portion 12, and deletes the print datathat has been stored before in the print data generation memory 11 f(Step S108).

On the other hand, upon acquisition of the print data outputted from theprint data generation portion 11, the printing portion 12 forms (prints)dots of sizes corresponding to the respective dot numbers on the printmedium using the black nozzle module 50 according to the print data thusacquired (Step S110).

For example, when the print head is of a type employing piezo actuators,a technical method of controlling the dot sizes can be readily achievedby controlling a quantity of ejected ink by varying a voltage applied tothe piezo actuators.

As has been described, by performing the error diffusion processingusing the diagonally weighted diffusion matrix on the pixel data in aportion where the banding phenomenon occurs due to a failing nozzle, aphenomenon visually acknowledged as a white streak or a dark streak canbe made less noticeable than the dot pattern forming results shown inFIG. 8 and FIG. 9.

In the first embodiment, the image data acquisition portion 10corresponds to the image data acquisition portion in the first aspect orthe twenty-fifth aspect. The N-value conversion processing to convertthe M-value pixel value of the selected pixel data to an N value by theN-value conversion portion 11 a and the N-value conversion informationstorage portion 11 b corresponds to the N-value conversion processingportion in the first aspect or the twenty-fifth aspect. The errordiffusion portion 11 c corresponds to the error diffusion portion in anyone the first, fifth, sixth, twenty-fifth, twenty-ninth, and thirtiethaspects. The nozzle information storage portion 11 d corresponds to thenozzle information storage portion in the first aspect or thetwenty-fifth aspect. The error diffusion matrix storage portion 11 ecorresponds to the error diffusion matrix storage portion in any one ofthe first, tenth, eighteenth, twenty-fifth, thirty-second, and thefortieth aspects. The selecting processing of the outstanding pixel databy the N-value conversion portion 11 a corresponds to the pixel dataselecting portion in the first aspect or the twenty-fifth aspect. Theprocessing to convert the data after the N-value conversion to dataindicating the dot number of the dot formed by the nozzle in the formatthat the printing portion 12 can interpret by the N-value conversionportion 11 a corresponds to print data generation portion in the firstaspect or the twenty-fifth aspect. The printing portion 12 correspondsto the printing portion in the first aspect.

In the first embodiment, Steps S102 and S104 correspond to the acquiringof the image data in any one of the tenth, eighteenth, thirty-second,and fortieth aspects. Steps S202, S206, and S208 correspond to theperforming of the N-value conversion processing in any one of the tenth,sixteenth, eighteenth, thirty-second, thirty-eighth, and fortiethaspects and to the generating of the print data in any one of the tenth,eighteenth, thirty-second, and fortieth aspects. Step S204 correspondsto the selecting of pixel data in any one of the tenth, eighteenth,thirty-second, and fortieth aspects. Steps S210 through S216, and S220correspond to the diffusing of the error in any one of the tenth,fourteenth, fifteenth, sixteenth, eighteenth, twenty-second,twenty-third, thirty-second, thirty-sixth, thirty-seventh, fortieth,forty-fourth, and forty-fifth aspects. Step S110 corresponds to theprinting in the tenth aspect or the eighteenth aspect.

Second Embodiment

A second embodiment of the invention will now be described withreference to the accompanying drawings. FIG. 15 is a view showing thesecond embodiment of the printing device, the printing device controlprogram, the printing device control method, the print data generationdevice, the print data generation program, and the print data generationmethod of the invention.

The configurations of the printing device and the computer system of thesecond embodiment are the same as their counterparts of the firstembodiment shown in FIG. 1 and FIG. 2, respectively. In the secondembodiment, the print data generation processing performed in Step S106in FIG. 5 of the first embodiment is replaced with the one in FIG. 15.

The generation principle underlying the print data generation processingof FIG. 15 is the same as the one in the first embodiment except thatthe selected pixel data corresponding to a nozzle having an ink ejectionfailure is not converted to an N value, and instead the value of theoriginal pixel data, which is deemed as an error, is diffused to thepixel data as diffusion targets in the neighborhood with the use of aspecial error diffusion matrix for avoiding the banding stored in theerror diffusion matrix storage portion 11 e. Hereinafter, only theportion different from the first embodiment will be described, and thedescription of the same portion will not be repeated herein.

Hereinafter, the print data generation processing in Step S106 in thesecond embodiment will be described in detail with reference to FIG. 15.

FIG. 15 is a flowchart detailing the print data generation processing bythe printing device 100.

This print data generation processing is the processing according towhich the followings are performed. Firstly, whether the nozzle Ncorresponding to the selected pixel data is a failing nozzle is judged.When the nozzle N is a failing nozzle that causes the flight deviation,an error is calculated by performing the N-value conversion. On theother hand, when the nozzle N is a failing nozzle having an ink ejectionfailure, the N-value conversion is not performed, and instead, the pixelvalue of the original pixel data is used as an error. Further, anappropriate error diffusion matrix for the pixel data corresponding tothe failing nozzle is selected from the error diffusion matrix storageportion 11 e according to the judgment result. The error diffusionprocessing is then performed using the error diffusion matrix thusselected. The print data is finally generated according to the imagedata having undergone the foregoing processing. When the print datageneration processing is performed in Step S106, as is shown in FIG. 15,the flow proceeds to Step S300.

In Step S300, the N-value conversion portion 11 a judges whether theM-value CMYK image data has been acquired from the image dataacquisition portion 10 is judged. Upon judgment that the CMYK image datahas been acquired (Yes), the acquired CMYK image data is stored in theprint data generation memory 11 f. The flow then proceeds to Step S302.Otherwise (No), the judgment processing is repeated until the CMYK imagedata is acquired.

When the flow proceeds to Step S302, the N-value conversion portion 11 aacquires the N-value conversion information by reading out N-valueconversion information from the N-value conversion information storageportion 11 b and storing the read N-value conversion information in aspecific region of the RAM 62. The flow then proceeds to Step S304.

In Step S304, the N-value conversion portion 11 a selects pixel datathat has not been subjected to the N-value conversion processing in theCMYK image data stored in the print data generation memory 11 f. Theflow then proceeds to Step S306.

In Step S306, the N-value conversion portion 11 a acquires nozzleinformation corresponding to the selected pixel data by reading out thenozzle information from the nozzle information storage portion 11 d andstoring the read nozzle information in a specific region of the RAM 62.The flow then proceeds to Step S308.

In Step S308, the N-value conversion portion 11 a judges whether thenozzle N corresponding to the selected pixel data has an ink ejectionfailure according to the nozzle information acquired in Step S306. Uponjudgment that the nozzle N has an ink ejection failure (Yes), the flowproceeds to Step S310; otherwise (No), the flow proceeds to Step S318.

When the flow proceeds to Step S310, the N-value conversion portion 11 aconverts the pixel value of the selected pixel value to the lowestdensity value (for example, the density value of “0” or the luminancevalue of “255”), and generates data indicating “no dot” corresponding tothe lowest density value in the format that the printing portion 12 caninterpret. The selected pixel data in the CMYK image data stored in theprint data generation memory 11 f is then updated to the data thusgenerated. At the same time, a difference between the pixel value beforethe conversion and the pixel value after the conversion is calculated asan error, and the error and the information about the selected pixeldata (including the information indicating the ejection failure) aretransmitted to the error diffusion portion 11 c. The flow then proceedsto Step S312.

In the description of Step S310 above, the pixel data corresponding tothe dot formed by the nozzle having an ink ejection failure wasconverted to the lowest density value indicating “no dot” in FIG. 10 byway of example. The invention, however, is not limited to thisconfiguration. The image in the printing result may be viewed locally,and when a dot size for an image in the converted portion is too smallto be perceived by humans, the pixel data may be converted, for example,to a luminance value in the range of forming a dot as small as thosewith the dot number 1 or the dot number 2 shown in FIG. 10 instead ofthe value indicating “no dot”.

In Step S312, upon acquisition of the error calculated in Step S310 andthe information about the selected pixel data corresponding to the errorfrom the N-value conversion portion 11 a, the error diffusion portion 11c selectively reads out the diagonally weighted error diffusion matrixfrom the error diffusion matrix storage portion 11 e. The flow thenproceeds to Step S314.

In Step S314, the error diffusion portion 11 c diffuses the error of theselected pixel data acquired in Step S312 or Step S322 with the use ofthe error diffusion matrix selected in Step S312 or Step S324 to thepixel data as diffusion targets in the image data stored in the printdata generation memory 11 f, and thereby updates the pixel data asdiffusion targets. The flow then proceeds to Step S316.

In Step S316, the N-value conversion portion 11 a judges whether theN-value conversion processing has been completed for all the pixel data.Upon judgment that the N-value conversion has been completed (Yes), thesequence of processing is ended and the device is returned to theoriginal processing; otherwise (No), the flow returns to Step S304.

On the other hand, in a case where the flow proceeds to Step S318because it is judged in Step S308 that the nozzle N corresponding to theselected pixel data has no ink ejection failure, the selected pixel dataselected in Step S304 is converted to an N value according to theN-value conversion information acquired in Step S302. Also, the valueafter the N-value conversion is converted to data indicating the dotsize to be formed in the format that the printing portion 12 caninterpret. The selected pixel data in the CMYK image data stored in theprint data generation memory 11 f is thus updated to the converted data.The flow then proceeds to Step S320.

In Step S320, the N-value conversion portion 11 a calculates adifference between the pixel value before the N-value conversion and thepixel value after the N-value conversion of the selected pixel dataaccording to the result of the N-value conversion in Step S318, andtransmits the calculation result as an error to the error diffusionportion 11 c together with the information about the selected pixeldata. The flow then proceeds to Step S322.

In Step S322, the error diffusion portion 11 c judges whether the nozzleN corresponding to the selected pixel data is a failing nozzle thatcauses the flight deviation according to the nozzle information storedin the RAM 62. Upon judgment that the nozzle N is a failing nozzle(Yes), the flow proceeds to Step S324; otherwise (No), the step proceedsto Step S326.

When the flow proceeds to Step S324, the error diffusion portion 11 cselectively reads out the diagonally weighted error diffusion matrixfrom the error diffusion matrix storage portion 11 e. The flow thenproceeds to Step S314.

On the other hand, when the flow proceeds to Step S326 because it isjudged in Step S322 that the nozzle corresponding to the selected pixeldata is not a failing nozzle, the error diffusion portion 11 cselectively reads out the normal error diffusion matrix from the errordiffusion matrix storage portion lie. The flow then proceeds to StepS314.

The operation of the second embodiment will now be described.

In the second embodiment, too, as is shown in FIG. 8 of the firstembodiment, dots formed by the nozzle N6 in the black nozzle module 50are displaced by a distance a toward dots formed by the normal nozzle N7immediately to the right, and as a result, a white streak occurs betweendots formed by the nozzle N6 and dots formed by the nozzle N5immediately to the left. Also, as is shown in FIG. 9, the nozzle N6 inthe black nozzle module 50 has an ink ejection failure, and dots thatshould have been formed are not formed. As a result, a white streakoccurs between dots formed by the nozzle N5 and dots formed by thenozzle N7.

In the printing device 100 according to the second embodiment of theinvention, the same processing as the first embodiment is performed onthe pixel data corresponding to the nozzle that causes the flightdeviation, whereas the N-value conversion is not performed on the pixeldata corresponding to the nozzle having an ejection failure, andinstead, the error diffusion processing is performed with the use of thediagonally weight error diffusion matrix by deeming the pixel valueitself as an error. The printing device 100 is therefore able to produceprint data that can make a white streak or a dark streak lessnoticeable.

In the print data generation processing in the second embodiment, theN-value conversion portion 11 a acquires the CMYK image data from theimage data acquisition portion 10, and stores the acquired CMYK imagedata in the print data generation memory 11 f (Step S300). Subsequently,the N-value conversion portion 11 a reads out the N-value conversioninformation from the N-value conversion information storage portion 11b, and stores the read N-value conversion information in a specificregion of the RAM 62 (Step S302).

Upon acquisition of the N-value conversion information, the N-valueconversion portion 11 a selects pixel data that has not been subjectedto the N-value conversion processing in the CMYK image data stored inthe print data generation memory 11 f (Step S304). The N-valueconversion portion 11 a then reads out nozzle information correspondingto the selected pixel data from the nozzle information storage portion11 d and stores the read nozzle information in a specific region of theRAM 62 (Step S306). Subsequently, the N-value conversion portion 11 ajudges whether the nozzle N has an ink ejection failure according to theacquired nozzle information indicating the absence or presence of anejection failure in the nozzle N corresponding to the selected pixeldata as shown in FIG. 13 in the first embodiment (Step S308).

Upon judgment that the nozzle N corresponding to the selected pixel datahas an ejection failure, the N-value conversion portion 11 a convertsthe pixel value of the selected pixel data to the lowest density value(for example, the luminance value of “255”), and generates dataindicating “no dot” corresponding to the lowest density value in theformat that the printing portion 12 can interpret. The N-valueconversion portion 11 a then updates the selected pixel data stored inthe print data generation memory 11 f to the data thus generated. Also,the N-value conversion portion 11 a calculates a difference between thepixel value before the conversion and the pixel value after theconversion (luminance value of “255”) of the selected pixel data as theerror for the error diffusion, and transmits the error and theinformation about the selected pixel data (including the informationindicating the presence of an ejection failure) to the error diffusionportion 11 c (Step S310).

On the other hand, upon acquisition of the error and the information(indicating the presence of an ejection failure) of the selected pixeldata from the N-value conversion portion 11 a, the error diffusionportion 11 c selects the diagonally weighted error diffusion matrix fromthe error diffusion matrix storage portion 11 f (Step S312).Subsequently, as is shown in FIG. 14A in the first embodiment, the errordiffusion portion 11 c diffuses the error to the pixel data as diffusiontargets at the lower left, immediately below, at the lower right, andimmediately to the right with respect to the selected pixel data (thedata of the pixel of interest in FIG. 14A) in the CMYK image data storedin the print data generation memory 11 f at the diffusion ratios set inthe diagonally weighted error diffusion matrix as shown in FIG. 14B,thereby updating the CMYK image data (pixel data as diffusion targets)stored in the print data generation memory 11 f (Step S314).

Upon judgment that the nozzle N corresponding to the selected pixel datahas no ejection failure, the N-value conversion portion 11 a convertsthe selected pixel data to an N value according to the N-valueconversion information thus acquired, and converts the value after theN-value conversion to data indicating the dot number of the dot formedby the nozzle for this value after the N-value conversion in the formatthat the printing portion 12 can interpret, thereby updating theselected pixel data in the CMYK image data stored in the print datageneration memory 11 f (Step S318). Further, the N-value conversionportion 11 a calculates an error between the luminance value before theconversion and the luminance value after the conversion corresponding tothe dot number of the selected pixel data, and transmits the error thuscalculated and the information about the selected pixel data to theerror diffusion portion 11 c (Step S320).

As with the first embodiment, the error diffusion portion 11 csubsequently judges whether the nozzle N corresponding to the selectedpixel data causes the flight deviation according to the absoluteejection accuracy information and the relative ejection accuracyinformation about the nozzle N shown in FIG. 11A and FIG. 11B,respectively (Step S322). Upon judging the occurrence of the flightdeviation (branching to Yes in Step S322), the error diffusion portion11 c selects the diagonally weighted error diffusion matrix from theerror diffusion matrix storage portion 11 f (Step S324). On the otherhand, upon judging that the nozzle N corresponding to the selected pixeldata is a normal nozzle (branching to No in Step S322), the errordiffusion portion 11 c selects the normal error diffusion matrix fromthe error diffusion matrix storage portion 11 f (Step S326).

Once the error diffusion matrix for the selected pixel data is selectedin this manner, the errors calculated are diffused to the pixel data asdiffusion targets at the lower left, immediately below, at the lowerright, and immediately to the right with respect to the selected pixeldata (data of the pixel of interest in FIG. 14A) in the CMYK image datastored in the print data generation memory 11 f as is shown in FIG. 14Aat the diffusion ratios set in the selected error diffusion matrix shownin FIG. 14B. The CMYK image data stored in the print data generationmemory 11 f is thus updated.

The print data generation processing ends when the N-value conversionand the error diffusion processing have been performed on all the pixeldata in the CMYK image data stored in the print data generation memory11 f (branching to Yes in Step S316).

As has been described, by performing the N-value conversion and theerror diffusion processing using the diagonally weighted error diffusionmatrix on the pixel data in a portion where the banding phenomenonoccurs due to the nozzle that causes the flight deviation, and byperforming the error diffusion processing using the diagonally weightederror diffusion matrix not by subjecting the pixel data in a portionwhere the banding phenomenon occurs due to an ink ejection failure tothe N-value conversion processing but by using the pixel value of thepixel data as an error, it is possible to make the phenomenon visuallyperceived as a white streak or a dark streak less noticeable than in thedot pattern forming results shown in FIG. 8 or FIG. 9.

In other words, in contrast to the first embodiment where the N-valueconversion and the error diffusion processing are performed to the pixeldata corresponding to the nozzle having an ink ejection failure on theassumption that dots are formed, in the second embodiment, the N-valueconversion is not performed on the assumption that dots will not beformed and the grayscale of the pixel data in a portion where a dot isnot formed is compensated with the pixel data in the neighborhood.Hence, the pixel value of the pixel data for which a dot is not formedis used as the error, and is diffused to outstanding pixel data in theneighborhood.

In the second embodiment, the image data acquisition portion 10corresponds to the image data acquisition portion in the first aspect orthe twenty-fifth aspect. The N-value conversion processing to convert anM-value pixel value of the selected pixel data to an N value by theN-value conversion portion 11 a and the N-value information storageportion 11 b corresponds to the N-value conversion portion in any one ofthe first, seventh, twenty-fifth, and thirty-first aspects. The errordiffusion portion 11 c corresponds to the error diffusion portion in anyone of the first, fifth, seventh, twenty-fifth, twenty-ninth, andthirty-first aspects. The nozzle information storage portion 11 dcorresponds to the nozzle information storage portion in the firstaspect or the twenty-fifth aspect. The error diffusion matrix storageportion lie corresponds to the error diffusion matrix storage portion inany one of the first, tenth, eighteenth, twenty-fifth, thirty-second,and fortieth aspects. The processing to select outstanding pixel data bythe N-value conversion portion 11 a corresponds to the pixel dataselecting portion in the first aspect or the twenty-fifth aspect. Theprocessing to convert the data after the N-value conversion to the dataindicating the dot number of a dot formed by the nozzle in the formatthat the printing portion 12 can interpret and the processing togenerate the data indicating “no dot” corresponding to the selectedpixel data converted to the lowest density value in the format that theprinting portion 12 can interpret by the N-value conversion portion 11 acorrespond to the print data generation portion in the first aspect orthe twenty-fifth aspect. The printing portion 12 corresponds to theprint portion in the first aspect.

In the second embodiment, Steps S102 and S104 correspond to theacquiring of the image data in any one of the tenth, eighteenth,thirty-second, and fortieth aspects. Steps S202, S206, and S208correspond to the performing of the N-value conversion processing in anyone of the tenth, sixteenth, eighteenth, twenty-fourth, thirty-second,thirty-eighth, fortieth, and forty-sixth aspects and to the generatingof the print data in any one of the tenth, eighteenth, thirty-second,and fortieth aspects. Step S204 corresponds to the selecting of thepixel data in any one of the tenth, eighteenth, thirty-second, andfortieth aspects. Steps S210 through S216 and S220 correspond to thediffusing of the error in any one of the tenth, fourteenth, sixteenth,eighteenth, twenty-second, twenty-fourth, thirty-second, thirty-sixth,thirty-eighth, fortieth, forty-fourth, and forty-sixth aspects. StepS110 corresponds to the printing in the tenth aspect or the eighteenthaspect.

On example of the invention will now be described with reference to FIG.16 through FIG. 19.

FIG. 16 shows a gradation image used in this example. FIG. 17 is anormal error diffusion matrix used in the error diffusion technique inthe related art. FIG. 18 is a view showing the result when the gradationimage of FIG. 16 is subjected to the quaternary conversion using theerror diffusion matrix by the technique in the related art shown in FIG.17. FIG. 19 is a view showing the result when the gradation image ofFIG. 16 is subjected to quaternary conversion using the error diffusionprocessing technique of the invention.

Firstly, the error diffusion technique in the related art will bedescribed.

According to the error diffusion technique in the related art, in a casewhere pixel data is selected sequentially from left to right in each rowof the image data, as is shown in FIG. 17, the error is diffused tooutstanding pixel data immediately to the right of the selected pixel,as well as outstanding pixel data at the lower left, immediately below,and at the lower right, that is, a total of four pixel data. In short,the error diffusion technique in the related art diffuses the error morein the downward direction of the selected pixel data. FIG. 18 shows animage of the gradation image after the quaternary conversion by thetechnique in the related art. When image portion 21 a through 21 c inthe gradation image after the quaternary conversion as shown in FIG. 18are closely observed, the kinds of dots (dots have plural kinds ofsizes) are not switched smoothly during printing in the image portion 21a because the diffusion direction of the error diffusion matrix used forthe quaternary conversion has a characteristic to diffuse the error morein the downward direction. This gives rise to a delay in switching dotsfrom one kind to another during printing. In particular, in a portionenclosed by a circle, dots are formed to flow in the diagonally rightdown direction. In addition, in the image portion 21 b (halftoneportion), dots of plural kinds forming this portion are not mixedhomogeneously because of the same reason in the image portion 21 a. Thisgives rise to deterioration of the image quality. In the image portion21 c, dots flow in the diagonally right down direction in a portionenclosed by a circle, that is, a so-called dot tailing phenomenonoccurs, because of the same reason in the image portions 21 a and 21 b.

Each of the phenomena occurring in the enlarged image portions 21 athrough 21 c deteriorates the image quality of the gradation image.

On the contrary, in the error diffusion processing method of theinvention, as with the embodiments described above, for the pixel datacorresponding to the dot portion formed by the nozzle judged as being afailing nozzle in the gradation image of FIG. 16, the diagonallyweighted error diffusion matrix is selected from the error diffusionmatrices shown in FIG. 14B, so that the pixel data is subjected toquaternary conversion through the error diffusion processing. For thepixel data corresponding to the dot portion formed by a normal nozzlejudged as not being a failing nozzle, a normal error diffusion matrix isselected from the error diffusion matrices shown in FIG. 14B, so thatthe pixel data is subjected to quaternary conversion through the errordiffusion processing. In short, the pixel data is subjected toquaternary conversion using the same error diffusion processingtechnique in the first embodiment. FIG. 19 shows the result of thequaternary conversion. When image portions 23 a through 23 c in thegradation image after the quaternary conversion shown in FIG. 19 areclosely observed, a delay in switching of the kinds of dots is improvedin comparison with the technique in the related art in the image portion23 a, and the dispersion property of the kinds of dots in the shadowportion is enhanced. The image quality is therefore enhanced incomparison with the image portion 21 a of FIG. 18. In the image portion23 b, plural kinds of dots are mixed more homogeneously than by thetechnique in the related art, and the image quality is also enhanced incomparison with the image portion 21 b in FIG. 18. In the image portion23 c, the dot tailing phenomenon in the high-light portion is improvedin comparison with the technique in the related art, and the imagequality is also enhanced in comparison with the image portion 21 c ofFIG. 18.

The printing devices of the first and second embodiments arecharacterized in that a special printing portion 20 is not necessarybecause the print data is generated from the image data to match withthe characteristic of the print head in the existing printing device byhardly providing modifications. Hence, the existing ink jet printer inthe related art can be used intact as the printing device of theinvention. In addition, by separating the printing portion 12 from theprinting device 100 in the embodiments above, the function can beachieved with a general-purpose printing instruction terminal (printdata generation device), such as the PC, alone.

An event addressed by the invention is not limited to the flightdeviation phenomenon. It goes without saying that the invention is alsoapplicable to a case where the ink is ejected in a perpendiculardirection (normally), but there is a discrepancy between the formationcontent of the nozzle and the normal position, and the formed dotresults in a phenomenon same as the flight deviation phenomenon.

The printing devices 100 in the first and second embodiments are notlimited to the line-head ink-jet printer, and it can be applied to amulti-pass ink-jet printer. In the case of a line-head ink-jet printer,it is possible to obtain high-quality printed matter in which a whitestreak or a dark streak is hardly noticeable by a 1-pass operation evenwhen the fight deviation phenomenon occurs. In the case of themulti-pass ink-jet printer, printing faster than in the related art isenabled because the number of reciprocations can be reduced.

FIG. 20A through FIG. 20C show the printing methods of the line-headink-jet printer and the multi-pass ink-jet printer.

As is shown in FIG. 20A, given the width direction of a rectangularprint medium (sheet) S as the main scanning direction of the image data,and the longitudinal direction as the sub-scanning direction of theimage data, then as is shown in FIG. 20B, in the case of the line-headink-jet printer, the print head 200 is as long as the width of the printmedium (sheet) S. Hence, by moving the print medium (sheet) S in thesub-scanning direction with respect to the print head 200 while fixingthe print head 200, the printing can be completed by the so-called1-pass (operation). Alternatively, printing can be executed, as with theso-called flat-head scanner, by moving the print head 200 in thesub-scanning direction while fixing the print medium (sheet) S, or bymoving the both in the opposite directions. On the contrary, in the caseof the multi-pass ink-jet printer, as is shown in FIG. 20C, the printhead 200 far shorter than the sheet width is positioned in a directionorthogonal to the main scanning direction, and printing is executed bymoving the print medium (sheet) S in the sub-scanning direction by apredetermined pitch while moving the print head 200 to reciprocate anumber of times in the main scanning direction. The multi-pass ink-jetprinter therefore has a disadvantage that the printing takes longer thanthe line-head ink-jet printer. However, because the print head 200 canbe positioned repetitively at an arbitrary position, of the bandingphenomenon, the white streak phenomenon, in particular, can besuppressed to some extent.

Also, the first and second embodiments described the ink-jet printerthat executes printing by ejecting dot-shaped ink by way of example.However, the invention is also applicable to other printing devicesusing a print head in which printing mechanisms are aligned in line, forexample, a thermal head printer called a thermoelectric printer or athermal printer.

Referring to FIG. 3, the nozzle modules 50, 52, 54, and 56 of therespective colors provided to the print head 200 are of a configurationin which nozzles N are continuously aligned linearly in the longitudinaldirection of the print head 200. However, as is shown in FIG. 21, eachof the nozzle modules 50, 52, 54, and 56 may comprise plural shortnozzle units 50 a, 50 b, . . . , and 50 n, so that these units arealigned at the top and bottom of the print head 200 in the movingdirection. In particular, by forming each of the nozzle modules 50, 52,54, and 56 using plural short nozzle units 50 a, 50 b, . . . , and 50 nin this manner, it is possible to shorten a distance from dot to dotsubstantially without narrowing the actual distance (pitch) between thedots in the respective nozzle units 50 a, 50 b, . . . , and 50 n. Theprinting device is thus able to address a high-resolution image withease.

The first and second embodiments described a case where the errordiffusion processing is performed by selecting a special error diffusionmatrix (diagonally weighted diffusion matrix) for avoiding the bandingphenomenon caused by the flight deviation or an ink ejection failure ofthe nozzle N by way of example. The invention, however, is not limitedto this configuration, and the invention can address other factorscausing the banding by preparing an error diffusion matrix appropriateto such a banding-causing factor, such as the y characteristic of dotsformed.

The first and the second embodiments described a case where the errordiffusion is performed using the error diffusion matrices having thesame size, the same number of pixels as diffusion targets, and the sameshape by way of example. However, the invention is not limited to thisconfiguration. Plural kinds of error diffusion matrices, includingmatrices having different sizes, matrices having different numbers ofthe pixels as diffusion targets, and matrices having different shapesmay be prepared, so that the size of the matrix, the number of thepixels as diffusion targets, and the shape of the matrix can be changedin response to, for example, a quantity of flight deviation.Alternatively, the error diffusion may be performed using a matrix of arelatively small size for the portion where the banding occurs.

In the first embodiment, the diagonally weighted error diffusion matrixis selected from a group of the same diagonally weighted error diffusionmatrices for the pixel data of a failing nozzle causing the flightdeviation or having an ejection failure. The invention, however, is notlimited to this configuration, and an adequate group may be formed foreach factor causing the banding phenomenon, such as a group for theflight deviation and a group for an ejection failure, so that anappropriate error diffusion matrix can be selected from the group of thecorresponding factor.

In the first embodiment, whether the nozzle N corresponding to theselected pixel data is a failing nozzle is judged, and when the nozzle Nis a failing nozzle, an error of the selected pixel data is diffused tooutstanding pixel data in the neighborhood using a special errordiffusion matrix. The invention, however, is not limited to thisconfiguration. The error diffusion processing using a special errordiffusion matrix may be performed by using the special error diffusionmatrix not only for the selected pixel data, but also for the pixel datacorresponding to the nozzles in the neighborhood of the nozzlecorresponding to the selected pixel data. The nozzles in theneighborhood of the nozzle corresponding to the selected pixel datareferred to herein include, for example, in the case of the occurrenceof a white streak, a nozzle that forms dots at a displaced position anda nozzle that forms normal dots spaced apart from such displaced dots bya larger distance than usual. In the case of a dark streak, such nozzlesinclude a nozzle that forms dots at a displaced position and a nozzlethat forms normal dots with a distance shorter than usual from suchdisplaced dots or a nozzle that forms dots that overlap partially orentirely on such displaced dots. The invention is not limited to thisexample, and the range of the neighborhood may be enlarged to includethree nozzles immediately to the left and right of the nozzle inquestion.

1. A printing device that prints an image on a medium with a print headhaving nozzles capable of forming dots, the device comprising: an imagedata acquisition portion that acquires image data having an M-value(M≧3) pixel value; a nozzle information storage portion that storesnozzle information indicating a characteristic of each nozzle; an errordiffusion matrix storage portion that stores plural error diffusionmatrices having different diffusion ratios of an error assigned to pixeldata as diffusion targets; a pixel data selecting portion that selectsspecific pixel data in the image data; an N-value conversion processingportion that performs N-value conversion processing to covert theM-value pixel value indicated by the selected pixel data to an N (M>N≧2)value; an error diffusion portion that diffuses the error to pixel datathat has not been subjected to the N-value conversion processing in theimage data according to the error diffusion matrix by using a differencebetween: a value of the selected pixel data; and a value of the selectedpixel data after the N-value conversion processing as the error, therebyupdating pixel values in the image data; a print data generation portionthat generates print data that defines information about a dot formationcontent of the nozzles corresponding to the image data after the N-valueconversion processing; and a printing portion that prints the image onthe medium using the print head according to the print data, wherein theerror diffusion portion selects a specific error diffusion matrix foreach selected pixel data from the error diffusion matrix storage portionaccording to the nozzle information, and diffuses the error to the pixeldata that has not been subjected to the N-value conversion processingaccording to the selected error diffusion matrix.
 2. The printing deviceaccording to claim 1, wherein: the nozzle information includes ejectionfailure information indicating at least one of an absence and a presenceof an ink ejection failure for each nozzle.
 3. The printing deviceaccording to claim 1, wherein: the nozzle information includesinformation about a quantity of position displacement between an actualforming position of a dot by each nozzle and a desired forming positionof the dot.
 4. The printing device according to claim 1, wherein: theerror diffusion matrix includes a diagonally weighted error diffusionmatrix that is an error diffusion matrix in which the diffusion ratio ofthe error assigned to the pixel data corresponding to pixels asdiffusion targets positioned in a diagonal direction, which is adirection other than a pixel selecting direction and a directionperpendicular to the pixel selecting direction, is made larger than thediffusion ratio assigned to the pixel data as diffusion targetspositioned in a direction other than the diagonal direction.
 5. Theprinting device according to claim 4, wherein: in the error diffusionprocessing for the error of pixel data corresponding to at least one ofa nozzle having a quantity of the position displacement equal to orlarger than a specific quantity and a nozzle in a neighborhood thereof,the error diffusion portion diffuses the error of the pixel data to thepixel data that has not been subjected to the N-value conversionprocessing using the diagonally weighted error diffusion matrix.
 6. Theprinting device according to claim 4, wherein: in the error diffusionprocessing for the error of pixel data corresponding to at least one ofa nozzle having an ink ejection failure and a nozzle in a neighborhoodthereof, the error diffusion portion diffuses the error of the pixeldata to the pixel data that has not been subjected to the N-valueconversion processing using the diagonally weighted error diffusionmatrix.
 7. The printing device according to claim 3, wherein: theN-value conversion processing portion converts the pixel value of thepixel data corresponding to a nozzle having an ink ejection failure to avalue close to one of a lowest density value and a lowest luminancevalue; and the error diffusion portion diffuses the error to the pixeldata that has not been subjected to the N-value conversion processing ina neighborhood of the pixel data after conversion using the errordiffusion matrix by using a difference between the pixel value of thepixel data before the conversion and the pixel value of the pixel dataafter the conversion as the error.
 8. The printing device according toclaim 1, wherein: the print head comprises a print head in which thenozzles are aligned continuously across a width which is at least aslarge as a placement region of the medium.
 9. The printing deviceaccording to claim 1, wherein: the print head comprises a print headthat executes printing while moving in a direction orthogonal to atransportation direction of the medium.
 10. A printing device controlprogram to control a printing device that prints an image on a mediumwith a print head having nozzles capable of forming dots, the printingdevice control program causing a computer to perform processing asfollows: acquiring image data having an M-value (M≧3) pixel value;selecting specific pixel data in the image data; performing N-valueconversion processing to covert the M-value pixel value indicated by theselected pixel data to an N (M>N≧2) value; diffusing the error to pixeldata that has not been subjected to the N-value conversion processing inthe image data according to the error diffusion matrix by using adifference between: a value of the selected pixel data; and a value ofthe selected pixel data after the N-value conversion processing as theerror, thereby updating pixel values in the image data; generating printdata that defines information about a dot formation content of thenozzles corresponding to the image data after the N-value conversionprocessing; and printing the image on the medium using the print headaccording to the print data, wherein when diffusing the error, aspecific error diffusion matrix is selected for each selected pixel datafrom an error diffusion matrix storage portion having stored pluralerror diffusion matrices having different diffusion ratios of the errorassigned to pixel data as diffusion targets according to nozzleinformation indicating a characteristic of each nozzle, and the error isdiffused to the pixel data that has not been subjected to the N-valueconversion processing according to the selected error diffusion matrix.11. A printing device control method for controlling a printing devicethat prints an image on a medium with a print head having nozzlescapable of forming dots, the printing device control method comprising:acquiring image data having an M-value (M≧3) pixel value; selectingspecific pixel data in the image data; performing N-value conversionprocessing to covert the M-value pixel value indicated by the selectedpixel data to an N (M>N≧2) value; diffusing the error to pixel data thathas not been subjected to the N-value conversion processing in the imagedata according to the error diffusion matrix by using a differencebetween: a value of the selected pixel data; and a value of the selectedpixel data after the N-value conversion processing as the error, therebyupdating pixel values in the image data; generating print data thatdefines information about a dot formation content of the nozzlescorresponding to the image data after the N-value conversion processing;and printing the image on the medium using the print head according tothe print data, wherein when diffusing the error, a specific errordiffusion matrix is selected for each selected pixel data from an errordiffusion matrix storage portion having stored plural error diffusionmatrices having different diffusion ratios of the error assigned topixel data as diffusion targets according to nozzle informationindicating a characteristic of each nozzle, and the error is diffused tothe pixel data that has not been subjected to the N-value conversionprocessing according to the selected error diffusion matrix.
 12. A printdata generation device that generates print data used in a printingdevice that prints an image on a medium with a print head having nozzlescapable of forming dots, the print data generation device comprising: animage data acquisition portion that acquires image data having anM-value (M≧3) pixel value; a nozzle information storage portion thatstores nozzle information indicating a characteristic of each nozzle; anerror diffusion matrix storage portion that stores plural errordiffusion matrices having different diffusion ratios of an errorassigned to pixel data as diffusion targets; a pixel data selectingportion that selects specific pixel data in the image data; an N-valueconversion processing portion that performs N-value conversionprocessing to covert the M-value pixel value indicated by the selectedpixel data to an N (M>N≧2) value; an error diffusion portion thatdiffuses the error to pixel data that has not been subjected to theN-value conversion processing in the image data according to the errordiffusion matrix by using a difference between: a value of the selectedpixel data; and a value of the selected pixel data after the N-valueconversion processing as the error, thereby updating pixel values in theimage data; and a print data generation portion that generates printdata that defines information about a dot formation content of thenozzles corresponding to the image data after the N-value conversionprocessing, wherein the error diffusion portion selects a specific errordiffusion matrix for each selected pixel data from the error diffusionmatrix storage portion according to the nozzle information, and diffusesthe error to the pixel data that has not been subjected to the N-valueconversion processing according to the selected error diffusion matrix.13. A print data generation program that generates print data used in aprinting device that prints an image on a medium with a print headhaving nozzles capable of forming dots, the printing data generationprogram causing a computer to perform processing as follows: acquiringimage data having an M-value (M≧3) pixel value; selecting specific pixeldata in the image data; performing N-value conversion processing tocovert the M-value pixel value indicated by the selected pixel data toan N (M>N≧2) value; diffusing the error to pixel data that has not beensubjected to the N-value conversion processing in the image dataaccording to the error diffusion matrix by using a difference between: avalue of the selected pixel data; and a value of the selected pixel dataafter the N-value conversion processing as the error, thereby updatingpixel values in the image data; and generating print data that definesinformation about a dot formation content of the nozzles correspondingto the image data after the N-value conversion processing, wherein whendiffusing the error, a specific error diffusion matrix is selected foreach selected pixel data from an error diffusion matrix storage portionhaving stored plural error diffusion matrices having different diffusionratios of the error assigned to pixel data as diffusion targetsaccording to nozzle information indicating a characteristic of eachnozzle, and the error is diffused to the pixel data that has not beensubjected to the N-value conversion processing according to the selectederror diffusion matrix.
 14. A print data generation method forgenerating print data used in a printing device that prints an image ona medium with a print head having nozzles capable of forming dots, theprint data generation method comprising: acquiring image data having anM-value (M≧3) pixel value; selecting specific pixel data in the imagedata; performing N-value conversion processing to covert the M-valuepixel value indicated by the selected pixel data to an N (M>N≧2) value;diffusing the error to pixel data that has not been subjected to theN-value conversion processing in the image data according to the errordiffusion matrix by using a difference between: a value of the selectedpixel data; and a value of the selected pixel data after the N-valueconversion processing as the error, thereby updating pixel values in theimage data; and generating print data that defines information about adot formation content of the nozzles corresponding to the image dataafter the N-value conversion processing, wherein when diffusing theerror, a specific error diffusion matrix is selected for each selectedpixel data from an error diffusion matrix storage portion having storedplural error diffusion matrices having different diffusion ratios of theerror assigned to pixel data as diffusion targets according to nozzleinformation indicating a characteristic of each nozzle, and the error isdiffused to the pixel data that has not been subjected to the N-valueconversion processing according to the selected error diffusion matrix.